If I had to buy one today (like if they took my T34 and my KV-5 away from me), the IS-6 or Super Pershing would both get strong considerations.
I'm encountering both more and more, and IMHO their main drawbacks stem from players not using them correctly. Occasionally I'll see either used correctly and am impressed upon the spot.
But if the big, dumb and armored behemoth is the play style you like (in other words, if the KV-1 fit your play style in tier 5 tanks more than the others), the T34 comes close to it if, again, played correctly.
With perhaps slight more than a modicum of talent, the KV-5, IS-6, Super and Type 59 all are great producers. Likewise for the 8.8 if you have the patience to be a non-turreted TD who stays behind the main force.