I don't think we should be allowed to use the fact that Spock Prime is coming after Nemesis. That sets the bar too high. I mean, the Borg were slicing the D like a chocolate cake, and it only survived because a god pulled them out of there. Khan is a formidable man, but he's no Borg.
True, I was limiting my list to movie villians. Toss in TV series villains and Khans goes from average (at best) to light weight.
Also, there's the exact phrasing of Spock Prime's words in that scene to consider:
"Khan Noonien Singh is the most dangerous adversary the Enterprise
Nimoy was clearly only referring to the "most dangerous" foe encountered during his own personal service as a crewmember of the various Enterprise
s in the 23rd Century, not to any of the later adversaries encountered by Picard and his crew in the 24th Century.
Being only the Prime Universe under discussion in that particular moment, Q and the Borg wouldn't really even enter into the scope of the conversation.