I hate to say it as I love how the new Sion sounds, but there are two champions who I find ultimately better in terms of voice and audio quality. Nasus' booming and deep voice compiled into an audio filter that makes it sound kind of echo-like is just fantastic for the champion that he is in terms of lore and his sheer size. Then, as much as he is a hated champion, Yasuo's voice actor (Liam O'Brien) did a fantastic job and is probably one of my favorite voice actors. When they brought him on to do Yasuo, I fanboy'd the hell out.
And maybe it's just me, but I don't think that there are that many female champions that quite reach the audio quality that most champions have been able to meet as of today's standards. Perhaps, if there is ever a VU or a rework for any of the female champions in the near future, we could see something done about it? Honestly the only two female champs that I believe meet today's standards are Jinx and Vi, and they're two of the most recent lady champs.