I think Serguei Lyakhovich has most potential. Talent-wise he has showed good beard and power, decent speed and he has some boxing skill as well.
Other arguments would be that he has excelent trainer that has brought him up, not joined with after succes like in most cases. He has hearth and wits about him(taking knee vs Brewster was very game) and he's motivated - he came to US with borrowed $100 and managed to make it this far. He's also married so hopefully his wify won't let him waste his talent by parting to much, hehe
All in all I see him only geting better with time, in 3-4 fights he might be ready to take on Wlad.
As for best fights I would love to see these:
Wlad/Rahman/Serguei vs each other
Tua vs Peter
I love watching Toney but I doubt we'll see him in decent shape again, he's just to fat to be active enough now :-/