I can't remember all the fights well enough to discern who I think is the easiest or hardest, so if it's ok I'll rant about boss fights in Mega Man 3 instead. After picking up the 3DS version, I set a save state at the Wily Teleport System room so I could finally try and learn how to properly fight all of them.
In order of what I feel is easiest to hardest:
-Top Man is by far the easiest. He may be the easiest Robot Master to beat period. He only has 2 attacks, and he never mixes them up to make dodging them tricky. It's simple to consistently beat him without getting hit after figuring out his moves, and he has a slight weakness to the Mega Buster so you can finish him off fast despite him being invincible while spinning.
-Snake Man is a Robot Master I surprisingly used to have a lot of trouble with, because if you don't know what to do you'll end up bumping into him a lot and losing tons of health. Even though I had seen a friend's video about how to beat him without getting hit, it wasn't until I got to repeat the fight with the 3DS that I finally got down the pattern. As long as you claim the middle pillar and use it to jump over Snake Man, you can pretty easily avoid ever getting hit by him. It's not hard to pull off either, so as long as you figure this out, Snake Man becomes an easy boss you can beat even with low health.
-Magnet Man has a really straight forward pattern, but also a lot of points where he's invincible. It can take awhile to beat him with just a Mega Buster, which means more time to mess up dodging his hops or his Magnet Missiles. Not too tough to beat, but you have to stay pretty active.
-Spark Man, and all the other Robot Masters below, I have a difficult time fighting without getting hit (even after recognizing their patterns). While his attacks are fairly easy to dodge as long as you're on the opposite side of the room as he is, it can be tricky not getting into that situation. He is capable of stopping at any of the 5 platforms, so if you're at the far end of one and he starts sparking on the platform next to you, you're in trouble. As long as you have enough health to take one or two hits though, you should be able to always take him out.
-Gemini Man has 2 reactionary patterns, and I'm not sure I understand the best way to take out the 2nd one yet. The first one is straight forward enough; jump over one of the clones, shoot, jump his bullet, repeat. After one of the clones is gone, shooting will make him jump, which allows you to simply walk under him and keep attacking. HOWEVER, after some time he will eventually fire the Gemini Laser, during which he will not jump to your shot and he'll actually change direction, something that I have a difficult time figuring out how to deal with. Again, as long as you can take a hit or two, beating him isn't too terrible.
-Hard Man's fight is interesting when it comes to discerning difficulty. If you had no health left, Hard Man's fight would be incredibly difficult, because his Hard Knuckles are not at all easy to dodge. If you have a good amount of health though, you won't have much trouble spamming the Mega Buster and finishing off Hard Man before his Hard Knuckles finish you. Most of the Robot Master's attacks in this game are simple enough to dodge IF you know that they're coming, but the Hard Knuckles are way tricky, which makes it much more tempting to spam Mega Buster fire and take the hits rather than waste time trying to dodge and failing.
-Shadow Man has a reputation for being one of the most challenging Mega Buster only Robot Masters to fight. Even though he only has two attacks like Top Man, there's a 50/50 chance he'll use one or the other after his set of hops, and the difference between them require split second reaction timing to properly dodge. When he slides, he does so immediately, to the point where if I'm not already jumping towards him right when he begins I'll end up getting hit and taking a lot of damage. However, jumping early can be a fatal mistake, as his other attack of throwing Shadow Blades requires that you be ready to jump after a small delay. Touching Shadow Man hurts a lot more than the Shadow Blade, so I tend to jump and pray that he'll slide more often than he'll throw Shadow Blades. For me, this makes the fight a gamble, as he could end up throwing tons of Shadow Blades and sap all of my health.
-Needle Man is a Robot Master who I never pegged to be so difficult in the past, but that's probably because his stage comes with a free E-Can and the Gemini Laser messes him up bad. With only a Mega Buster and a single health bar, though, Needle Man is very deadly. He can shoot off needles high and low, hop short or far, and has a short range physical attack that is super painful. Even if you focused entirely on dodging without a thought for having to fight back, you'd have a hard time not getting snagged by one of his attacks. The way he mixes up his moves makes him very unpredictable, so you have to be ready for anything, all the while trying to tag him with shots for the few times he's on the ground. It's fitting that the Robot Master with spikes is so dangerous to Mega Man!
)? Honestly I thought he was pretty easy. I usually don't even attempt to dodge his Hard Knuckle and just spam the Mega Buster, I only move when he does his ground pound. For the most part I can drain his health faster than he can drain mine.
So from easiest to hardest it goes:
