This is a great thread, Cody.
After doing some serious scouring of my collection and photobucket, here are some of my favorites, and why I like them.
I know this has been cliched to death, but I have always and will continue to like this set. When I first saw these, I loved them. I like the base line design on the side, the photography was something we really hadn't seen before 1989, the glossy finish, and the backs were even cool too. UD did a great job with their first set.
This is actually a reproduction of the first game-used set issued by UD. The originals sold for a ton of money when they first came out. I like this design, it's simple and makes clear to the first time game-used card holder, exactly what they are holding in their hands.
Of course I had to show some 2005 Diamond Kings! This may be my favorite game-used/auto subset of all time. I think the frames add something very nice to the cards, and there are different colors (red/green/blue). The design is rather simple, but it incorporates the portraits of the players, has the game-used pieces on the side and then has a nice space for the auto at the bottom.
06 Bowman Chrome was a very pretty set. I loved trying to put together the rainbow set for my Brandon Wood collection. The on-card autos look sharp on here. It's a basic design, but it's good to keep it that way so the design and the different refractor surfaces don't get all garbled up.
The Tools of the Trade Patch set has been one of my favorites in both baseball and football. They've stuck to pretty much the same design in every set, but the jumbo patches and even regular patches are in my mind, the nicest patch cards you can get. Not to mention those jumbo Babe Ruth jerseys from 2005 Update are some of the most sought after game-used cards in the industry.