There are many strong opening options, but there are even more weak ones. I often see people making mistakes within the first 2 rounds of a game, so I thought it would be useful if I talked about what I consider in an opening.
Step 1: Deciding on an Economic Plan
First of all, you have to decide how much and what kind of economy you want to play. Do you want to play a pair of Mana Crystals? Do you want to use a Battle Forge to play discount rings? Do you want to use Lair + Meditation Amulet? Do you want to just use discount rings? There is a limit to how much economy you can safely get away with against aggressive opponents, so if you want to do more than 2 rounds of economy, you'll need contingency plans in case you need to divert from playing economy, but the first step in coming up with an opening is deciding on which economy you want to use. You can always get away with one round of economy, and you can get away with 2 rounds of economy in most circumstances. 3 rounds of economy is not something you can do every game, and if you want to try something of this nature (such as an Anvil Throne Warlord with Barracks, Battle Forge, and Meditation Amulet), you'll have to spend more time thinking about contingencies to deal with early aggression. Teleport/Divine Intervention turbo rush, Wall of Thorns -> push spam (especially against a Wizard because of Wizard's Tower), and attack spell spam being the things to look out for. Double move -> Lair can also be pretty threatening. Look out for opponents that double move on turn 1, especially if they don't cast much. Even against most aggressive openings, you won't be under much pressure until round 3.
You generally want to get your economy down first because of the large starting distance between you and your opponent, and cards that generate mana and actions are better the earlier you play them.
Mana Crystals/Flowers are good opening plays for builds that want to play many level 3+ creatures, creature Spawnpoints are good for builds that want to play many level 1-2 creatures, and Battle Forge is good for builds that are running relatively few creatures. A few of the more efficient Spawnpoints from a mana generation perspective are good at playing higher cost creatures, such as the Gate to Voltari.
Spawnpoints are more about generating an action advantage than a mana advantage. A Mana Crystal will break even faster than most Spawnpoints from a mana perspective, but a Spawnpoint lets you get creatures down without tying down your mage.
Once you have a rough idea of what kind of economy you want to play, you should optimize it as much as possible. Don't throw down just a Garrison Post turn 1, then play a Barracks turn 2. It's much better to play Barracks and Garrison Post on the first turn than to spread it out both from a mana generation and an action efficiency perspective. How can you increase your effective channeling as fast as possible, and take advantage of any discounts as efficiently as possible?
In terms of placement, you generally want to place Mana Crystals/Flowers as far away from the enemy as possible. Creature Spawnpoint placement varies quite a bit. The Barracks and Vine Tree should be placed in your starting corner because they both have ways of extending their range, and are relatively fragile against fire attacks. The Lair should be placed in either NC (near center) or FC (far center) depending on how aggressive the opponent is being, whether you want to play another card this round, and how aggressive you want to be. The Lair is extremely durable, and placing it in one of the central zones gives better board control and allows you to apply pressure more quickly and efficiently. The Temple of Asyra is a bit more fragile, and tends to be used in less aggressive strategies, so it should usually be placed in your starting corner or one zone away from that. I generally like to play the Gate to Voltari one zone away from my starting corner to get slightly better board control while still maintaining some distance between it and my opponent. I'm not really a fan of the Graveyard, Samara Tree, or Pentagram, and I'm not sure about optimal placement for them. The Battle Forge should be placed one zone from your starting zone for defensive openings and in NC or FC for aggressive openings. You want to make sure you'll have the reach for the Battle Forge to cast equipment on your mage, so keep the range of equipment in mind when placing it (0-2).
Don't Harmonize a Battle Forge. It's not worth it. I try to avoid using Harmonize in general. I only use it if it will make the difference between being able to consistently cast creatures with the Spawnpoint and not being able to cast creatures consistently. While it makes the Spawnpoint more mana efficient, it also makes it a much more tempting target for attacks.
Step 2 for Economic Openings: Defense and Counter Pressure
The next thing to consider for economic openings is defense and counter pressure. Cards that reduce incoming damage make early aggression much weaker. It's easy to plan for these types of things if you opened with a Battle Forge, as it can be used to play equipment that give Armor. Defenses are another possibility, but they tend to be expensive enough to disrupt your opening more than the cheap Armor equipment like Leather Gloves and Leather Boots. Brace Yourself is a cheap option that can fit into many openings that makes you much more resilient against early burst damage. Remember that enchantments that give Armor can wait to be revealed until your opponent has rolled the attack dice, and you know whether the Armor will actually reduce the incoming damage.
Cheap walls, Tanglevine, Force Hold, Turn to Stone, Block and Teleport Trap are other options to consider for slowing down early aggression.
Counter pressure is the ability to threaten an opponent that threatens you. For example, zombies are bad at applying quick pressure to a passive opponent due to Lumbering, but they can tear through the forces of a quick attack by non-Flying creatures. Similarly, Iron Golems are fantastic guards that can win a straight up slug fest with anything of similar cost, but they can’t apply any early pressure against a passive opponent. Guards can both reduce the damage coming at your mage, and wear down your opponent's threats, so are usually a good option against aggression. A good guard has efficient stats, particularly health and Armor, and a reasonable counterattack. Defenses help too. Timber Wolves, Orc Butchers, Iron Golems, Dwarf Panzergaurdes, and Guardian Angels are some examples of good guards.
Particularly when your opponent is being more aggressive than you are, always think about how much pressure you are under, and what you can do about that. Project forward a turn, and think about how much pressure your opponent can apply next turn. Use hindering to your advantage to limit the number of attacks your opponent can get against you.
Step 2 for Aggressive Openings: Pressure
If you want to put your opponent under a lot of pressure early, think about how you want to put them under pressure, and how you can follow up with even more pressure if they can withstand your initial attack. I tend to think that Divine Intervention/Teleport turbo rush plays that attack on turn 2 are weak because they put some pressure on their opponent on turn 2, but their ability to apply pressure going forward is drastically reduced. Plus they are pretty telegraphed if you know what to look for. If you can take out their initial threat, they are usually in a lot of trouble.
Battle Forge rushes are a popular choice for aggressive play. You sprint to NC in the first round, then play a Battle Forge in far center. Round two is often playing a big creature, an Enchanter’s Ring, and a face down enchantment. There are many variations that can work. One of the reasons it’s so good is that the Battle Forge can be used to boost offense or defense as required, which makes a rush less vulnerable to counter pressure. In addition, the Battle Forge gives enough economy that you won’t fall behind an economic opponent nearly as quickly if they manage to draw out the game.
You generally want to get a creature within the first 2-3 rounds. For aggressive builds, you generally want to get a creature round 2. Round 1 is often used to play a cheap economy card like a Mana Crystal, discount ring, or Battle Forge and to get closer to the opponent.
When evaluating an aggressive opening, it’s important to think about how resilient your threats are. Grimson Deadeye Sniper can give you full board coverage turn 2 with Rouse the Beast, but he’s also vulnerable, and can be killed very easily. A Steelclaw Grizzly is much tougher, but has to get closer to the enemy mage to do anything.
General Considerations
Don’t send in creatures unsupported. I’ve seen it happen with creatures ranging from Goblin Grunts, Feral Bobcats, and Bitterwood Foxes to Steelclaw Grizzlies. A lone creature without support can often be killed or disabled before accomplishing anything, particularly if you make mistakes in your sequencing. If you activate a Steelclaw Grizzly to double move into the zone with my mage and 2 Knights of Westlock before any of my creatures activate, it’s entirely possible I could kill the Steelclaw before it ever gets an attack in.
In terms of the execution of your opening, unless there is an advantage to using your quickcast early, save it until later. The less information you give your opponent, the better. Playing a discount ring right before casting a spell that will benefit makes sense, but playing a Battle Forge at the start of a round does not. Using your quickcast early on is often of great value once you've actually engaged, but when neither player is applying direct pressure, it's more of an information game.
When playing against a holy mage, particularly the Priestess, keep in mind the possibility of your opponent using Divine Intervention to counter one of your spells. If they counter a turn 2 Lord of Fire, you've basically already lost. I'll consider switching a Battle Forge rush around in this matchup in order to get the creature first and the Battle Forge second so that I can get my essential cards up without interference.
There are some other types of long term investments like Altar of Skulls that do nothing for you early, but can have devastating effects in the late game. If you want to focus on one of those types of investments, consider what the optimal time to play the card is. In the case of Altar of Skulls, there is no way for it to make any progress round 1, so consider playing an Acolyte round 1 and Altar of Skulls round 2.
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