What is your favorite role that Minorin voiced that isn't Nagato Yuki? I'm sorry to exclude that one, but I can guess Yuki would probably overwhelm every other option there is (although I have an idea what role would take its place already). Also, if you think I left out any really really important roles, let me know and I can add it, because even I (sadly) have not watched every series that Minorin helped voice in.
Oh screw it, I'll add in Nagato Yuki because I'd like to see the other answer battle it out.
I had to vote Yuki out of sheer truthfulness, but Iwasaki Minami and Minami Chiaki are both very high up there.
Don't blame Minorin for playing the worst character in Tenjho Tenge. It was her first major role, and therefore didn't get much choice in the matter... but I agree, her voice fit the character in that it was out of place with the rest of the anime... :P
I always wonder how good Minorin would do it now if Tenjou Tenge gets picked up again for a new season and she gets to reprise Aya. If that happens, I'd also worry about the possibility that they'd want her to mimic exactly how she did her voice before.