Gradius Collection for the PSP was amazing. The only Gradius game I had played before was the original, but now Gradius II and Gradius Gaiden are my favorite games in the series. II > Gaiden > I > IV > III. I should get around to playing V.
Castlevania: Dracula X Chronicles is a great remake of Rondo, but the retooled version of Symphony of the Night is a little meh.
The PS2 port of Tales of Symphonia is awesome. The addition of new ougis, new unison attacks, better battle menus, and the better controller were all welcome additions for fans.
On a similar note, the PS1 port of Tales of Phantasia is excellent. It basically improves upon everything in the original AND everything in the PS1 version of Tales of Destiny. The Pluto summon and playable Suzu are just icing on the cake. My one qualm is the nerfing of the Gremlin summon.
The PS2 port of Viewtiful Joe is a mixed bag. It's sad that they had to remove the cool Nintendo stuff flying around everywhere, but Dante might even be more fun to play as than Joe himself. The load times also seem a little longer.
Mega Man Maverick Hunter X and Mega Man Powered Up are both excellent. One goes the badass, plot-driven route and the other goes the chibi, kiddie route, but both new presentations completely fit their respective games. And the extras on both of them are WTF awesome.
The PS1 port of Mega Man X3 is pretty lame. The anime cutscenes do NOT improve over the original in-game cutscenes, and the new audio is universally inferior to that of the original.