Block Quote
M6
M4A3E8 Sherman
KV-1S
T34-85
VK30.01(P)
VK30.01(D)
VK36.01(H)
Cromwell
There are a few ways to answer this.
If you want the strongest-performing tanks at tier VI, look at Jylpah's excellent stats site, Blitzanalysiz(). Out of your list, the VK36.01(H) is the strongest at the moment, followed by the M6, T-34/85 and Cromwell. The other tanks on your list are performing in line with the skill of their players.
If you want to learn new things, many of these tanks have a particular skill you need to master to do well in them. If that comes naturally to you, you'll perform well in them from the start, but if not, then they will take longer to get the hang of but you'll improve your overall game skill in the process.
M6 - same as T1 Heavy. Hull-down, use gun depression, sidescrape.
M4A3E8 Sherman - Hull-down using excellent gun depression & strong turret, hide your sides and rear from HE, angle your front armour if exposed, match the angle of your front plate to terrain features.
KV-1S - Peek-a-boom, learn to hull-down with poor gun depression, use mobility to set up shots ("rotating"
, learn the derp life (terrible gun handling with high alpha and very long reload; when to use HE ammo).
T34-85 - mobility, hull-down with moderate gun depression. A rather nice all-rounder.
VK30.01(P) - don't have it, can't comment on playstyle.
VK30.01(D) - using angles to bounce off relatively thin armour. Also a good all-rounder.
VK36.01(H) - angling? This is just a really strong tank, not sure how much you learn playing it.
Cromwell - if you struggled with the Crusader, you'll struggle with this, which means you can learn something important from it. This is all about using gun depression, mobility and sneaky tactics to avoid getting shot while still getting as much of your high DPM out as possible using a rapid-fire, low-alpha gun. From memory, compared to Crusader, Cromwell has worse pen tier-for-tier but better mobility... it's been a while since I played either, though. British guns tend either to the pew-pew (Cru, Crom, Cent I, Caer, FV301) or excellent accuracy and marvellous HE (Cent 7/1, Conq). Likewise they tend to either have no mobility or gun depression but heavy armour everywhere (Church, BP), poor mobility but good gun depression and a strong turret with weak hull (both Cents, Caer), or excellent mobility and gun depression and no armour at all (Crom, FV301).