Glass looks great on females. So does Daedric, course Daedric looks great on males too. Ebony also looks great on males. I just don't use either one b/c they're too over the top for me, at least for Skyrim's feeling. I think I could've used the Daedric in Morrowind, and the Ebony for Oblivion though. Elven also looks great. I was very impressed with it in Oblivion, and even more so now in Skyrim.
I dunno, so far I really like just the standard thieves guild armor. It looks realistic, practical, comfortable. Overall though, I don't think I could pick one as the absolute best. All depends on what character I'd be playing.