Super Hydorah

+1 Recommendation for Super Hydorah, one of the best 2D shooters I've played, and I've played a lot of them. It may be helpful to know that this is an expansion for a freeware title 'Hydorah'. Original (Non-super) Hydorah has a great complete campaign that I'd recommend if you want to try this game, but with a few less planets between you and the end boss. You can grab it here from the developer's website.

Super or Original, Hydorah is a shooter that expects you to git gud, but it is more forgiving than most 90's arcade titles it imitates. It also gives you a decent number of saves, but will expect you to ration them.
Graphics, sound and music are fantastic.
10/10 game imo.