For me Familiars are about saving actions and a bit of mana.
So right now I tend to lean towards Nature's (Fellella, Pixie Familiar) and the Dark's (Sersiryx, Imp Familiar).
1 mana enchants even if you don't use them that round are golden and both of these familiars can take advantage of this. I am also looking at the Warlords familiar for this purpose as he is one of the most Mana starved mages IMO.
Agreed. Familiars tend to drain a lot of your mana, so I find they're best used if your mage intends to engage in melee (or perhaps bow/boomerang) combat. That is, a Familiar lets you get a quickcast (mage) and full cast (familiar) while attacking (mage's main action). That's why a Galvitar Forcemaster gets a big benefit from [mwcard=FWC16]Thoughtspore[/mwcard]s, and why [mwcard=MWSTX2FFC11]Sersiryx, Imp Familiar[/mwcard] and [mwcard=MW1C13]Fellella, Pixie Familiar[/mwcard] can be great with a hands-on Warlock or Beastmaster, respectively (although I find Sersiryx tends to draw a lot of hate and die quickly). And as littlenog mentioned, Sersiryx and Fellella can be used to buff your mage with Enchantments while your mage does the attacking, which really plays into the hands-on strategy.
In principle, the War school familiars ([mwcard=MWSTX2FFC07]Gurmash, Orc Seargeant[/mwcard] and [mwcard=FWC04]Goblin Builder[/mwcard]) should be good with the Anvil Throne Warlord for the same reason, but since a lot of Command spells require a Soldier target, and the Builder is designed for an Outpost and/or [mwcard=MWSTX2FFC12]Talos[/mwcard] strategy, and since a lot of mana goes into fueling Battle Forge to arm your Dwarven Juggernaut, you don't often see the War Familiars.
And in theory, [mwcard=FWQ09]Sectarus, Dark Rune Sword[/mwcard] should be good, since it is by definition a Familiar that works with a melee mage... but somehow it tends not to be great, probably because you have to plan the Curse spell in the Planning Phase and might have to attack a creature that you didn't want to put that particular Curse on (e.g. plan [mwcard=MW1E24]Magebane[/mwcard] or [mwcard=MW1E19]Ghoul Rot[/mwcard] but then have to attack a level 1 Guard). So it's a little tricky to use since the spell must target the defender of the melee attack. I've had some success with it, but it's limitations make it difficult/awkward to use, and a back-up [mwcard=MW1Q14]Lash of Hellfire[/mwcard] gets used almost as often.