Mage, silly.
Warrior would probably be second, and I've really been liking my hunter (finally, after about 6-7 rerolls). Fun to pvp with.
It seems that for me, a lot of whether or not I decide to keep a character is how much I like their story/persona. For instance, being a small, bearded fireball of a gnome in sweet goggles is just pure fun. Each of my characters seems to have their own... personality? Not sure if that's the word I'm looking for. I enjoy playing the characters that I mesh well with in that way. For instance, Flame is an intelligent engineer, loves to joke around with fun items in-game, even during raids. In Wrath, my main was Redletters, who was a strong, moral person, willing to be the first one into a fight, and put himself in harm's way so that others may succeed.
I don't get that into it with characters that aren't my main - but there's just something about some class/race combos that don't work for me, and I've found that this can lead to me losing interest in those characters. For instance, I don't have any dwarf characters. I like dwarves, and have tried to make a dwarf character many times. But for some reason, they never quite get off the ground. I've done well with humans, gnomes, and orcs.
In fact, here are my current class/race combos, with grades according to how much I like them:
Flamethrower - 85 Gnome Mage (A+, I don't think I'll ever get tired of being this)
Proginoskes - 85 Human Warrior (A, love the fury warrior playstyle, and humans are always win for me)
Mogadore - 82 Orc Warrior (A, again, a fury warrior leveled horde pre-cata just for the experience)
Redletters - 82 Human Paladin (B. Was an A in Wrath, but the playstyle changed so much and I don't really like it now)
Choppermad - 83 Night Elf Druid (C. I don't like Night Elves, but if I wanted to try druid, I had to. Don't especially like any druids specs currently, mainly because I prefer fire mage to balance, rogue to kitty feral, warrior to bear feral, and I don't like healing.)
Mumford - 71 Orc Hunter (A. Loving the hunter playstyle now that I'm getting used to it, and orcs are just cool.)
Marinus - 82 Human Death Knight (C. I don't care for the playstyle. I'm not into diseases and gross-sounding stuff like "blood-caked blade", etc. I leveled him during the novelty phase of Wrath, but since then his only XP has come from mining.)
Attarius - 82 Draenei Shaman (B. Draenei are ok, but I haven't played shaman in so long, I'd need to relearn. Also not wanting to level through Cata again. I really don't know why. This is why many of my characters are stuck in the low 80s.)
Avelade - 71 Human Rogue (B. I enjoy playing this one from time to time. My favorite part of this class is those quests where you have to kill one guy in the back of a cave, or room of a house somewhere. I enjoy stealthing in, sapping, blinding, shadowstepping through enemies, killing the one I want, and vanishing with no one knowing what just happened. I mean, I know they're just NPCs, but it's still fun. Incidentally, this is my only female character. I don't ever create characters that aren't the same gender as me, but for some reason, this one just felt right.)
Likeminded - 81 Human Priest (C. I love this guy. I love how he looks, love his name, and the gear set I got him [select Silver Thread pieces with a skull mace and black mageweave headband]. I don't like any of his possible specs. I don't like to heal, and don't really like shadow. He hangs out in SW not doing a whole lot.)
Dòóm - 20-something Dwarf Warlock (B. My final attempt at leveling both a Dwarf and a Warlock. I like him so far, but DOTs aren't really my style. I feel really clever about the 'angry eyes' in his name, and that's probably going to keep my interest for a while.)
So there's a little insight as to my attitude towards my characters.