Undead Warlock
I had a twink 49 undead warlock shortly after TBC came out and it was a beast. Was completely OP in PvP: once a buddy of mine (also undead warlock) took on 8 alliance at lumber mill in AB, killed every single one of them, took the flag and then defended. Both of us popped all our cooldowns, soul stones etc. but frigen 2 v 8!!
Also, warlock is a complete beast in PvE - can easily take on 3 monsters at once (when I was grinding up to 49 after TBC was released, and the warlock was very imba, I was literally able to pull 7 monsters at once, all with syphon life, coa, corruption etc.). I also figure that in the event that I am getting ganked left and right, I could spec demonology and go soul link to be practically unkillable even 2vs1.
Overall, I'd say my reason for Warlock is ease of leveling. I took an arms warrior from 1-60, pre-TBC, and it took friggen for-ev-er. I think I dinged 60 at like 12 days play time, although I did a lot of pvp, leveling blacksmithing. Following up on ease of leveling, I certainly don't have the same amount of time available to play WoW as I did 15 years ago and would like to get to end game sooner rather than later.