Which Knight of Cygnus has top DPS?

Well, three of 5 Cygnus classes have an innate critical skill. Soul Warriors don't, but they are probably the strongest solo 1v1 attacker, so it's hard to say. Flame Wizards don't even come close and are the only class that can be completely discounted.

It's really difficult to say without exact data on how fast the skills are and how certain skills work.

We have enough information about Windbreakers and Soul Masters, because of the timed delay on Wind Shot/Magnum Shot and Brandish's speed from Heroes. However, we lack Final Attack Speeds, which could always be assumed to be the same as Slash Blast/Power Strike. It's considerably fast, and it definitely contributes to their damage output, as opposed to hindering it like Slash Blast's does. Advanced Combo, not sure what data we have on that so far.