Ok here's my list...more or less in order sure.
10. Iguana Regrowth: It's good for keeping your creatures alive longer and getting the bang for your buck out of them. On smaller creatures it means they've got the resilience to be a long term factor. On a bigger creature it means they're going to cost the opponent even more resources to deal with.
9. Ballad of Courage: I'm using it more and more. I've been combining it with Enchantment Transfusion(trick I learned from Biblofilter) and getting excellent mileage out of it. It turns any creature into an instant real threat. Also good for getting through a Druids vine markers. I'd be amazed if Force Masters didn't make good use of it too.
8. Eye for an Eye: This is how a Paladin kills a resilient creature he just challenged. Holy can shrug off damage better than any other school and this is where it's at right here. Take the hit, hurt them with it, and then heal back. It makes everyone think twice about chucking that boulder and forces them to focus in with big hitters....that are then hurt and easily killed.
7. Nullify: It's in pretty much every book. There is nothing fancy or complicated about it. Just saves your butt and wastes their time.
6. Critical Strike: The days of super armored mages are over. There are so many ways for me to ignore your armor now a days. Obviously Warlords with their Vorpal Blades and at cost pricing make use of this but they also get value with their Spiked Bucklers. Almost every other mage has a piercing weapon too these days. Sires Trident, Beast Masters Hunting Spear, and even a Warlock can put on a mask and have 4 Piercing.
5. Magebane: Such a hassle and such a pain in the butt for the enemy mage. It amazes me how long people will let this stay on them. It kills your for being a mage. Push comes to shove it wastes a Dispel and hurts them a little.
4. Theft of Life: This is so friggin funny. I stopped Dissolving Regrowth Belts since this came out. I might as well get some of that action right? Shuts down or at least slows down a lot of peoples natural abilities and makes them pay mana for my healing. Also who wastes a Dispel on it?
3. Bear Strength: I cast this crap on everything! Timber Wolves become main threats, main threats become nightmares. My mage lives with it on him most games. Even smaller creatures are now "have to deal with" threats. Hitting people saves you mana, this makes hitting people more attractive. Consider it an alternative fuel source
2. Marked for Death: I swarm a lot obviously. My primary book is a Straywood and my secondary is an Adremelach Warlock who gets double benefit from this card. It cancels Aegis and forces a Dispel to come out. In my swarm books it makes Falcons serious threats and increases the aggro game greatly.
1. Brace Yourself: My number pick because I don't like getting hurt. I like to make this my only armor early on or when I have only 1 other armor. It has saved me more times than I can count. Turns Boulders into punches and can quickly thwart a sudden rush. Against swarms of zombies and other weaker creatures they're hoping for crits only. It's like a cheap poor mans Resilience.