Well if Sharp Dressed Man (my theme song as a youngun... I was only like 3 or 4 when that song came out and I was ALWAYS in suits. I wore out my "dressy clothes". Suits, vests, ties.) is a favorite now. Then I would suggest the following, as I've been a ZZTop fan since I was like 3-4 years old and I'm 28 now.
Eliminator. Great Classic album. Lots of great stuff in there. This was in my walkman CONSTANTLY in the late 80s. I wore out my copy in the early 90s.
Afterburner. Another great album, lots of fun. Eliminator and Afterburner fit together well.
Recycler - the album with Doubleback - but a lot of good fun songs on there.
Fandango - An earlier album with a live bit and some recorded stuff. Jesus Left Chicago is a great piece from that record
Tres Hombres - a lot of blues, a bit of boogie, and good rock.
If you're into the heavy RAWK then yeah stick with their later ones. Antenna, XXX, Rythmeen. Its beer and cigarette soaked rock n roll. But if you're finding you like the blusier and boogie side, then start trollin' their back catalogue. El Loco - WHICH HAS THE BOYS IN SOMBREROS! Or the aforementioned Deguello.
I'd also pick up the ZZ Top's Greatest Hits - its found nearly EVERYWHERE even though its close to 15-20 years old by now. I think that came out in like 92? Its the one with the pretty girls on the cover. But its got a nice selection of everything from Recycler on the way back. A little bit of everything. I'm not as impressed with their newer stuff, but its still rockin'. I enjoy it. Vincent Price Blues and Hummbucking Pt.2 off of Rythmeen is pretty good.
Honestly go to eMusic.com - get a subscription, you can download quite a bit of their stuff from there. For $15 bucks I think you get like 35 downloads so that's still a better deal than iTunes. But if you've got a used music store in your area, check them out. Recycler, Afterburner and Eliminator were all pretty big hits and nearly everyone had a copy of at least the the last two at one point or another.