Alpine CDA-7998, just because that's what I have. I think it's the best HU for my needs, but might not be for everyone else's. See where I'm going with this?
Alpine in general makes great HUs, as does Eclipse, Pioneer, and Clarion. The best HU is what makes it BEST for you... Ergonomics, features, EQ, time correction, etc, etc, etc...
I will end this here...