WotLK was the best by far I think for arena, specifically s7/s8. There will never be balance in the game but I would say that s7/s8 was the best we're gonna see regarding fun/skill and with such a huge variety of comps and fast paced play, people even spammed 2s even though you couldn't get a title in those seasons (and took it seriously).
This absolutely, s7/s8 was closest to balance wow has ever seen. Late Cata would have been there if pve trinkets were removed, rogues were toned down (hero class entire expansion), and mmr abuse issues were fixed.
The problem comes with the speed of the game. The faster the speed of game (MOP feels fast compared to tbc) the more chances for mistakes. Couple that with Mongo comps/abilities and most people are only using 75% of their class abilities anyway. How many of these top tier god comps do the druids bother to sooth? How often do you even see cross cc now?
You get to a point where the game is moving so fast (mostly b/c absurd CDs) that clean play is not as crucial.