Zeta Flow

Scorpio is my favorite level of all time. It came out when I was first learning the game, and I remember playing it and playing it and playing it, each time getting a little closer to victory, until finally I defeated it. That was a good feeling.

Of course later I discovered keyboard move and mouse aim, which makes it easier to beat levels like that... but that's still the level I go back and play the most often.

Thanks for reminding me about Space Pirate, too; that's a good one. Haven't had time to play the other ones you listed.

Here are a few from my own bookmark list:

Multi Leg Combat Vehicle This is a very large boss, and can be frustrating at first, but once you get the hang of it, it's fun. Also, one of the best visual designs I've ever seen.

Whrait Ship - This is a fairly early level. Takes some fast maneuvering to beat unless you get lucky.

INRI - Another early level. Good design and good gameplay, though some might find the visual design sacrilegious.

Octospider - Another good visual design.

Deathspider - An even better visual design; one of the best I've seen, IMHO. This is a pretty hard level. Watch out for the swinging head! Unfortunately the next level is a real letdown.

I came across this one today and had to add it to the "Worth A Replay" list:
The quantum and nuclear Base by Secret

I used 54 lives to finish it and I'm pretty sure a lot of you guys could do better But it is fun to play and it's a long level.

It's probably not for everybody but it is challenging and beautifully done. Kudos to Secret

They are great levels.