Once again getting my inspiration from FARK, here's a great post at Stuck in the 80s. It's an old debate: Which is the better band - Van Halen or Van Hagar? But the Stuck in the 80s blog is a great forum for the debate.
Personally, I think the author cheesed out by not coming right out and taking sides. He says that he likes both but if you pin him down he'll say old Van Halen. He backs this up in his top 5 VH album picks by only picking one Van Hagar offering.
My vote is for Diamond Dave-era Van Halen. I honestly don't even think of Hagar-era VH as the same band; which is why so many of us call them Van Hagar. A commentor on the 80s blog said that had Dave not left, the band would have broken up. I wonder if that might have been the best idea in the long run.