Mega Man 1 is a stupid game to begin with and going buster only does you no favors, so it's definitely that one. Doing everything else that you're allowed to do by going buster only (Bubble Lead, Crash Bomb, etc.) is manageable.
Mega Man 2 is not hard, all of the bosses have easy patterns (Crash Man, Quick Man.) Heat Man definitely takes practice, and Air Man is very difficult to do without getting hit (though not actually difficult, if that makes sense.)
Wave Man is one of the easiest bosses and I fight him first in Mega Man 5.
NES Shadow Man takes a lot of practice but remember to maintain your distance and be prepared for a long fight, Spark Man can also be super annoying due to the terrain of the room. Gemini Man is easy with enough practice
Mega Man 4...the ones that take practice are probably Drill Man, Ring Man. Dive Man, remember to avoid doing high jumps because you'll get fucked over by torpedoes or Dive Man moving under you.
Mega Man 7, I don't bother with the Wild Coil, it's too annoying to use. Final fight is a pain anyway.
Mega Man X6 from the X series is the obvious candidate from that series. If you ramp up the difficulty for X5 things get painful, especially certain boss fights