Personally, most of them weren't too hard once you figure them out, but there's one in particular that i have come to absolutely HATE, and that would be Lobsided, it is so unbalanced that unless you spends weeks straight just leveling your weapons, well, to quote Sans; You're gonna have a bad time.
Fiesta forever, hands down. Mostly because I still haven't beaten it. You don't get random weapons, so there is no way to make yourself stronger accept for the tank upgrades, and even if you are good you basically have to rely completely on luck. At least the final boss can be beaten with enough skill, I have tried fiesta forever like 20 times now and I have only gotten really close two times. And that's after looking at the guide and the forums for tips. Alright mini-rant over. I'm gonna keep trying though because I know I can beat it with enough tries and determination. (Yes I'm undertale trash too. :3)
From what i've heard, Fiesta Forever is only infuriating because it's pure luck where they hit, making it where you could be so close just to be fucked by randomly directed pellets.
For fiesta forever, it's probably best to just move your tank over one of the enemy tanks so that enemy friendly fire can help you a bit in dealing damage. It also helps to land your pinata beacons directly on the center of the tanks, since it gives a greater chance of more shots hitting them.