Aeroblasters is amazing. And a perfect arcade port. The version I have for MAME is identical. Any differences, if any, are minor.
As for difficulty in shmups, the harder the better. I love Galaga 88 on MAME, because like its predecessor Galaga, it is extremely difficult. However, Galaga 90 is not nearly as difficult. In fact, when it arrived in the mail I immediately put the game in and beat it in 15 or 20 minutes easily. I have never beat the game before on MAME or in the arcade. But I don't regret buying it as the graphics and music are pretty sweet. But Galaga 90 is missing the weird aliens talking in Japanese sequences. My point is that Galaga 90 would be sooo much better if it was a lot more difficult.
Raiden is Raiden. If you like tough shooters then this is your game. Kicks my butt everytime, but I still love it.
hehe
Gate of Thunder on Hard or Devil is pretty sweet, heck even Normal gets pretty challenging. I'm still working on it. I've stepped up to Normal and once I beat it on that I'll go to Devil....I'm frightened.
But right now Summer Carnival 93 Nexzr Special is where it's at for me. Excruciatingly difficult even on the easiest setting. Plus it has plenty of menu options (i.e. "stretching screen" or "acade mode", level select and live select. Though you can only select the level you have actually completed.) Your fighter in Nexzr is my favourite looking ship in any shmup. The pulsating green thrusters are sweet. The Carnival modes are interesting enough, but the real action is in the actual game. The only thing I regret about the Carnival version of the game I have is that it is missing the animated cut-scenes. But at least I have Youtube for that.