of the fighters on that list they're all on a similar level, probably go with Kovalev, Bivol, Beterbiev in no certain order. A Sober Kovalev and Beterbiev would be a great fight
I think the best Russian fighters never even hit the pro game.. Oleg Saitov was a 2X Olympic champion at WW.. Boris Lagutin was a 2X Olympic champion too... brilliant MW.. Nikolay Korolyov was a 9X USSR HW champion.. hed have been a force! Countless names I could mention.. but for the ones that did turn pro id say Bivol & Pirog.
Beterbiev for me. The win versus Prime Bivol is more impressive. Lets not forget Artur was 39 and not on his prime. Bivol needs to do more, either stay in Light Heavyweight and defend his Title, or go up Cruiser.
Beterbeef for me. Bivol may surpass him in resume, but he will never surpass him h2h. People will talk about Beterbiev for 100 years.
It's all recency bias with these things. If the poll had been done right after the first fight Beterbiev would have been ahead., or if Kovalev was in his prime now he'd be ahead. For me it's out of Kovalev and Beterbiev, and I'm going with Kovalev - he was an absolute monster in his prime years.