Skilgannon fought Decado(his reborn) in Swords of night and day and aknowledged that decado was better, sure you can say that Skilgannon wasn't used to his body, but then you can say Decado didn't have the mental powers of the ice killer. So Skilgannon is out the picture. Mental powers probably pwn Dace too. Qin Chong could beat him, but Qin Chong isn't actually human, so I am not sure he counts.
Waylander can beat anyone, true Druss is highly perceptive, but he has never faced an assassin of Waylanders calibre, just read the start of Hero in the Shadows, the best prologue ever IMO. The only one who could possibly beat Waylander is Shannow
who is Rolynd which gives him an important edge. Although he has guns which isn't really fair, same with Viruk, who's Zhi bow is just mean.
But Pagan will always hold the highest rank of badassery for beating the hell out of 9 joinings. A feat the likes of which can never be replicated.