Skynyrd's first 5 albums rank up there as one of the best 5-straight album runs of all time. They didn't put out some crap first and then hit their stride, they hit the ground running with the debut album and ran for 5 incredible albums.
ZZ Top started a little slower, although their first 5 albums were all good as well, with each one being better than the previous for the first four. Tejas was a step backwards in my opinion.
Anyway, I think the only truly fair way to judge these two bands is by their work in the 70's for the reason that Skynyrd was done in late 1977 after the plane crash and didn't do anything again until the early 90's.
Based on entire careers with both bands, I have to take ZZ Top based on their 70's and 80's work. IF we went with just the first 5 albums from each (as they started around the same time period), then I have to take Skynyrd in a landslide.