KatieHal came up with this idea first, and it is a good one. It doesn't necessarily have to be the absolute best and worst costumes, it can be more of a discussion about the various costumes in the series.
I never particularly liked Valanice's KQ7 costume. But I think Edgar looked cool at the end of KQ7--The tunic and cape and whatnot. Some of Conor's outfits are really cool, though I wish they would've stuck with the Medieval Fisherman looking version of him.
Graham's outfit IMO is the best of course. I'm not really sure of what to think of Alex's in KQ6--Sometimes it looks cool to me but others it looks rather ridiculous. As for villains, it's a toss up between Lolotte, Mordack and Alhazred. Manannan is your standard dark wizard, Malicia is much akin to a female Disney witch type, and Lucreto...Lucreto is interesting, I will give him that. He probably comes in second place.