As a new player? Conjurer is probably the most straightforward and easiest to grasp, while Ice Elementalist is probably the strongest overall. Summoners on the other hand get a spellbook with spells so good you can get runes just with them alone.

Wizards are interesting to play but aren't really a pure spellcasting class as your only damaging spell is horrible (but hey, at least it always hits) and you have to melee your opponents. Venom Mages are good until they run into something resistant to poison. I've heard Poison Arrow, the final poison spell, is actually pretty good but I never used it.

In between are Fire Elementalists, which are almost as straightforward as Conjurers but have the problem of relying on fire, which can give you troubles against the few rF+ enemies you encounter. Earth Elementalists are nice until you run into something with good AC (orc warriors come to mind), though LRD can be really good... sometimes. Air Elementalists are... you know what? There are too many mage starts IMO, if you compare it to other playstyles.

Transmuters are really more fighters than mages, with an added bonus of forms. Remember, it's not only Blade Hands, all of them are actually useful for something, so try them out. Skalds are distinguishingly bad, Warpers aren't much better, Enchanters can be fun but only on certain species and Arcane Marksmen are just worse Hunters. Also the above four aren't mages at all.

Strong starts include GrEE, because it's a gargoyle, which is cheating in itself; DEFE because it's a deep elf and FE tends to keep his opponents away from melee range; IE of any race decent at offensive magic. By the way, if you ever want to play a felid, FeCj might be the easiest one.