Best Retro XBLA Games That Aren't on PS3

[isiolia]

That (and the D&D ones) have disc versions to import on PS3, to boot.

[Nemoide]

One hidden gem I'd recommend is Trouble Witches Neo. It's one of the many shmups on the system and is extremely solid in terms of gameplay. It's a modern style bullet hell shooter but it has a novel "slow down bullets using magic" mechanic that I had a lot of fun with.
Just be warned that it has some of the worst English dialog ever recorded for a game...

The vast library of shooters for the 360 have caused it to kind of get forgotten, but it's a really good one!

[prfsnl_gmr]

Also...be sure to remember the classic Rare games on the 360, such as Banjo Kazooie, Banjo Tooie, and Perfect Dark.