Going to have to go with Paragon; to me, Evil/Renegade Shepard isn't really bad; (s)he's more a jerk. I understand what being a Renegade is supposed to be all about, but when you actually are a Renegade in Mass Effect, you come across as just being a massive prick rather than either an unconventional or an at-all-costs type. I mean, I like when Shepard is a jerk to jerks (Oh hello insinuating reporter *FALCON PUNCH!*), but I don't want to be a jerk to everyone just to get my Renegade up. The fact there is no benefit for a middle ground doesn't work for me; if Paragon and Renegade had nothing to do with intimidation and persuasion, I'd be fine with being bit of both.
I preferred the KOTOR system which meant there was some kind of middle ground which was of benefit (No discount either Light and Dark Force Powers, but no additional cost for using the opposite ones to the side you were aligned with.)