Tricky Kick is pretty hard. I put a lot of effort into Aero Blasters and I don't recall ever beating it. I find Vasteel hard because it crashes, does that count? I feel like actually beating Devil's Crush would be hard but I've never taken a realistic shot at it. And yeah, gonna go with Raiden to round out my top 5.
Deep Blue I've managed to get several loops through. It's much easier when you realize your primary goal is just to stay at the top of the screen and not die.