Gigantic Metool:
This thing is INSANE! It jumps, causing damage when you're on the ground, and if you even TOUCH it, you die instantly even with God Mode! What IS this thing? Awesome, that's what!
Walking Gamma:
Unlike the original NES versions of this boss, this one CAN MOVE. Not only that, but you fight him on the ground! The concept of a walking instakill boss is awesome, and the fact that very few weapons will have ANY effect on him is a painful blessing, because it means Wily's stepped up his game. Anyway, overall I loved it to hell!
Actual Spoiler: Opinion On Limited WeaponsI haven't tested every weapon, but I find Hard Knuckle to be very effective. Also, be sure to hone your jumping skills, because I found a few parts where I found jumping over the spike pit to be SAFER. Also, using the Top Spin when he's down makes this AWESOME. Tested the QUICKBEAM entity and the IGMETALBLADEWEP. Both had no effect. Nice.
EDIT 5/11/15: Oil puddles make Gamma slip instantly. Nice touch!
Evil Robot:
Fighting this guy in space was cool. You only have one weapon and one hope of victory: Dodge every attack, and when he's close enough to hit during his charge, use your charged attack. This guy's an awesome pain to beat. At least you're able to fly during this.
Genesis Unit:
Three robots trying to beat your butt off? Bring it on! The creators made this fight even better by giving them combined attacks! The fact their health is combined kinda makes no sense to me, but whatever, this fight is the best I've seen.
EDIT 5/11/15: Maybe when even just ONE of them is weak, they're too tired to do the formation correctly and manage to get the other two tangled up. Is that the idea? I think it is.
Feel free to post your opinions on these below!