Terezi definitely has some of the best themes imho. Terezi Owns, BL1ND JUST1C3, Terezi's Theme all top tier. Vriska's are all ****ing awesome too, as is Eridan's. I have a special little fondness for some of Gamzee's too~
Sollux has some pretty great ones too!
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Ryoko
http://img.imgcake.com/starryokojpgpy.jpg