Dudes...
TBC was the sweetspot simply, cause of the Arena, creating an esport and competition(temporary). I remember even humble random bgs much more active and enjoyable, too.
It has nothing to do with the raiding here. There was Black Temple and especially Sunwell for the hardcore raiders and even the most casual endgame player could visit Kara 10 mann and have some joy.
It was simple, it was less about farming lfrs and lfd, cause they did not exist, there was hard content, but most of the time you did only see it with dedication to a raiding guild and most importantly the Arena provided the first and last most innovative gameplay PvP players ever faced in wow history up to date. AFTER it nothing gamebreaking was ever released.....NOTHING at all, certainly do not count machines and rbgs.
So, TBC is most easily the sweetspot, doesn't take rocket science here to figure this out.