To be honest I wouldnt like to bet on things,
Druss perhaps the greatest warrior, Decado the greatest swordsman, Shannow the greatest Duelist and Waylander the greatest assassin. If they were to go head to head, not sure who would walk out, without weapons, more than likely Druss, with weapons then its a free for all dependent upon who was taken out first.
decado i think. and maybe waylander. he's more deadly because he could sneak up on you. in a fair fight, druss, but i think waylander wouldn't get to that position, he'd take you out before hand. *shrug* i also like him more than druss.
Druss is the greatest warrior in my opinion. The best swordsman would be Dace or Decado - its hard to say which. Most deadly would be Waylander, due mostly to the range he has to his advantage. I wouldnt put Shannow in the same category due to the fact he uses a gun. Although Viruk uses a zhi bow and he'd be pretty lethal as well.
Here is a wide one for thought. In Swords of Nightand Day we had the tinker Stavut, better known as Stavi. He controlled a pack or 40+ Joinings, who would folow his bidding. Piss him off and the rest of your life would not feature much tomorrow.
Deadliest would be Waylander tbh, for the simple reason that he does not fight fair...at all. One minute you are walking/sleeping/eating/rutting next minute there a small feathered black shaft in your eye.
Best swordsman would be that bloke who possessed Yu Yu in Waylander 3. Reckon he would slice Decado up nicely.