Probably gonna be a minority here, but I find Decarabia to be a really cool theme, especially the reversed parts.
Vincent, Nervous Longma, Gruff Life, and both Rotis themes are also among my favourites, but I feel like Plox's themes - especially Night Time Problem Plox - have to take the cake here.
Shame Krane doesn't have a theme (or if he does, I haven't noticed it in the background; there's none named after him at least). I think something with very quiet organs in the background would fit - and it plays the stereotype of him being a vampire. :)