Coolest: It's hard to decide between Darth Vader and Kahn. Darth has the mask and the scary breath, but now we know that he was once cute and precocious, which either makes him far less cool over all, or far more badass because he went from "hit the nose!" and "it's working, it's working!" to "Apology accepted, Captain Needa." So, yeah, Darth Vader.
Vilest: Whichever it is, it's got to come from
Farscape, because those bad guys just made you cringe whenever they were on the screen. It's hard to choose between Scorpius (killed 10,000 slaves on the off-chance that he might find the one he could use to ransom for Crichton's galaxy-destroying wormhole kowledge), Grayza (two words: heppel oil), or Furlow (apart from being just plain gross, she was willing to
sell galaxy-destroying wormhole tech to the highest bidders).
Stupidest: Just look to the Batman movies post-Keaton. Carrey's Riddler, Arney's Freeze, & Uma's Ivy were all just... stupid. But, obviously, Bane takes the taco.