I'm gonna give you my opinion, based on the fact that I have played every expansion ranging from TBC-Legion. TBC pvp is fun for 2 minutes. The best classes by far are rogues and druids, which you can even see from the 2s ladder. Casters run oom in about 2-3 minutes. I agree with the people that say some classes are missing key abilities, it just is true. I am usually top 3 3v3 on WoTLK pretty much every season I play in. It is, by far the best pvp expansion ever released by blizzard. People that say "DK , RET OP WTF!!111" are people who are probably basing their opinions on duels. No, DKs and Rets are not overpowered (they are really good in duels, battlegrounds), arena wise (3v3 based, not gonna mention healers) ret pals can play with hunters, warriors as far as viable compositions go. DKs can play with warriors, hunters, elemental shamans (although really hard comp to pull off at high rating),shadow priests etc, but they are definitely not OP. The reason it is the best expansion is because everything you play is pretty much viable and you can't say that anything hardcounters anything. Ofcourse some teams/classes do better against some and are worse against some but there is no "impossible matchup". I really suggest you play WOTLK and if your interest is just pvp, then play on blackrock, see how things go with bgs, duels etc and if you like it.
For any other questions please message me in my inbox I dont keep track of these threads