Metal Torrent on dsiware. It's a 5 stage score attack, but I've had it since release and I'll play through it ever other day or so. Two different ships, and the enemy formations can be set to patterns, or random. One of my top 5 downloadable games.
TwinBee is going to be released as one of the "3D Classics" in the near future. I don't think it was ever released in the US before, but it's sequel, Stinger, was a pretty good shooter on the NES. I'm certain it will be better than Xevious, which is still pretty mediocre IMO even with the 3D upgrade.