These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Best Boss Ever: The fight with Seven Force is widely regarded as one of the most memorable moments of both games.
Breather Level: Level five of Gunstar Super Heroes. A ride down a shaft, a run across a flat level plain (with possibly dangerous gas vents that are harmless as long as you bother to shoot them before going past), a none too difficult miniboss, a space shoot 'em up with two more easy minibosses, and a quick section shooting bad guys while standing on the top of your ship. Even on Hard you've got no one to blame but yourself if you dip below 50 health....
Moon 4. It has plenty of difficult challenges that must be completed in order to advance, and exploration is determined by a game of dice - complete with face number four being replaced by "back" after the first roll. To add insult to injury, some places send you to another room if you happen to fall down a chasm.
Visual Effects of Awesome: While dated by today's standards, the 3D polygon-like effects on some of the bosses was advanced for the time (especially since it arguably looked better than Star Fox on a system that's weaker than the SNES).
All the sprite rotation and scaling may not seem like much until you remember the Genesis lacked Mode 7.