Igor Vysotsky can claim to have been the greatest Soviet HW when it really mattered: beating a prime Teofilo Steveson, TWICE.
I've never followed the amateur code enough to confidently state who "the greatest" is or even to have a personal favourite. However, the name that jumps out for me when talking about Soviet HW's would be Igor Vysotsky. How much he achieved in competition I don't know but he's famous for being the bogeyman to the great Teofilo Stevenson. ESB even had an interview with the man himself: http://www.eastsideboxing.com/news.php?p=9323&more=1
That Clark was something else for a couple of years, he knocked down Greg Page twice in an Olympic Trials victory.