Ultimate Ghosts 'n Goblins looks just plain fabulous!
I don't know if I'd necessarily say that the PSP is the best system ever for 2D games (whether 2D sprites, or the so-called "2.5D" using 3D-rendered objects with primarily 2D movement like UGnG), but I'd say that it's probably one of the most ideal platforms to release these games on.
There are more powerful systems than PSP out there, which have the technical potential to provide even better versions of those games. For example, just about any currently-marketed home video game system that you could connect to your TV could do absolutely brilliant 2D games. However, I don't know how well a game like UGnG would sell on a current home console, because in recent years the market has dwindled for games like that on home consoles. They're still deemed "acceptable" on handhelds, and the PSP is the best of the bunch from a technical perspective, so that's why the PSP is such a good platform for these games.