Back during 3E D&D's run the full-color hardback became pretty standard for RPG rulebooks and even for supplements for the biggest games, so most of them are pretty cool looking now. 4E D&D brought in the matte-cover-with-a-glossy-insert look and that's cool. The 5E PHB looks pretty similar so nothing revolutionary there.
The Mutants and Masterminds 10th Anniversary Edition is a pretty cool looking book so I'll go with that as one entry. Blog entry here.
Ptolus, the big city book from Monte Cook a few years ago was another impressive book, both in the amount of content and in the presentation and organization of that content. I haven't really written about it much on the blog - I should probably do that.
- Last Unicorn's Star Trek The Next Generation rulebook - it was the first full-color hardback I remember seeing and it was full of images taken right from the series. It made a very strong impression at the time and I think it looks as good as a lot of newer books. Blog post here.
- Kenzer's Aces & Eights - a faux-leather cover like the old Time-Life western series and full-color interior with a lot of Remington artwork really conveys the flavor of the period.
- Paizo's Pathfinder Beginner Box is another impressive product and I think it looks better than any of the D&D starter sets, including the 5th edition one. Blog talk here.