Tier 5 - Pz-III/IV, M4, T-34 (but only with 57mm).
Tier 6 - M4E3E8, M4A3E2, 300x series, I prefer 3001H, but 3601 seems to be everyone's favorite.
Tier 7 - T20, KV-13 (horrible grind but great tank when fully upgraded), 3002DB..
I agree with the bolded, although 3001P has a terrible stock grind
Btw to specify: With Cromwell you HAVE a viable derpgun, but then you have to play it as a mobile direct fire artillery and ahoot at long range at stationary targets (to hit their top armor), for everything else... I have to say I enjoy Cromwell more with 6pdr than with 75mm HV.
I find it quite flexible when you've mastered the shell arc - at longer range you can even use autoaim to fire indirectly over small hills, which is fun. Close range is of course a happy place for derp guns, but you'll have to find weaker armour for the measly 95mm.
T32 or AMX50100 or Ferdi or Caernavon?
I prefer Mediums but I also do ok with IS and KT, JP or SU-152
128mm on ferdi made me giggle, it's very nice at that tier. Been a while since I played it though, and there is more penetration about these days