I would also stick with Loopz list, but for a kind of fun hard (pulstar is not a fun hard at all - just sick hard), I suggest the second loop of Sonic Wings 3. It's really not a great shooter, but the second loop is pretty difficult.
Shooters I've played, ranked from hardest to easiest:
Viewpoint
Pulstar
Last Resort
Blazing Star
Sonic Wings 3
Sonic Wings 2
Ghost Pilots
Interest factor:
Blazing Star - what can I say; I'm a fan of Blazing Star. Great music, great graphics, great control. Unlike Pulstar, you don't feel like you're flying through thick pea soup.
Viewpoint - one of the best soundtracks for any neo geo game, complemented by awesome pre-rendered graphics, and insane diffiulty. Not for people that suffer from claustrophobia.
Sonic Wings 3 - I like jungle, and I like character selection. I also like verticle shooters. The story cuts from character to character are also pretty funny. Worth playing just for that.
Sonic Wings 2 - see SW3.
Pulstar - not a great game at all. Just for some poser "r-type" fans. Real R-Type fans play R-Type, not Pulstar.
Last Resort - excellent soundtrack. Graphics are dated, and I really want to like it, but when I play it it's like "blech," so I just go to the sound test and listen to the music.
Ghost Pilots - looks really bad. I'd rather play Xanac or 1942 on the NES for an hour than Ghost Pilots for 10 minutes.