Wow, this is tough ... especially since both bands are SO different from each other. I love the vast majority of both bands catalogs.
Recently, I've been listening to a ZZ Top's albums from the 70's. I missed out on a lot of years of NOT listening to their first two albums. I've had them for a long time, but I really didn't spin them too often, and usually considered Tres Hombres as their first good album. I couldn't have been been more wrong. Their first two albums are littered with great, lesser known tracks.
Anyway, I don't know if I can vote. Will have to think about his for a while.