With the benefit of hindsight and speaking from PvE (solo, small and large group play) side of things only.
Classic: 8/10
BC: 9/10
Wrath: 7/10 (story saves it, else I'd put it at 6)
Cata: 6/10 (t11 saves it, else I'd put it at 5)
MoP: 7/10 (sofar, might eke out an 8 by the end if they keep removing all the crap decisions they made at the start)
Story is also very bland for me if I wasn't feeling forgiving I'd start it at a 6 with possibility for a 7 by the end.