Good can be somewhat subjective but is also not especially relevant to hacking. You can do some fantastic work in a level editor, and you can make an unplayable (even ironically*) mess with assembly hacking.
*personally I consider the kaizo mario hacks rather boring as these things go but I can also see their merit. Similarly there are also those that enjoy figuring out whatever abstract thought process the maker of a work was going with (see many a cryptic crossword through arguably adventure games to day many arty games). At the same time the base game would generally be designed to both teach the player the game and be playable by most people of the day at some level; you could happily assume people already know how to play and might be up for more of a challenge, or indeed a different type of challenge (megaman for me is basically a memory test, not sure how you could change it for a reaction test but it seems within the scope).
Level editors by and large mean you are limited by the base game. Some might include a select few additional hacks worked out by other means for you to include.
Graphics and some aspects of stats can be done more readily. Music on something this old tends to be harder.
Assembly is a type of programming, often considered quite a difficult one, and for many the end boss of learning ROM hacking as it is what allows you to ultimately brush up against the limits of the system itself for what you can change (thought practical limitations, that being "is it easier to just write my own game from scratch**?", will mean that stops rather before then for most things). However designing new enemies***, new mechanics, fixing bugs, probably a lot of music and more will be in the domain of this.
**this would include working on another game in the series if you can recreate whatever mechanics and port whatever assets you want from 2 into that.
***technically you could probably overwrite the frames of an enemy, maybe even change the hitbox ( https://www.pcgamer.com/how-hitboxes-work/
), without it but anything truly new would need something here.
I would also note that megaman is a somewhat popular franchise so in addition to whatever level editors are out there ( https://www.romhacking.net/utilities/490/ https://www.romhacking.net/documents/525/
, have not tried either myself and they look fairly normal for level editors I have seen for other things but that means little) you will probably also find some nice commented disassemblies in various states of completion. This is a serious perk over having to do this yourself for while assembly is rightly considered hard compared to a lot of other programming then 6502 (what the NES uses) or z80/8080 (if this is for the GB/GBC games) assembly is not so bad when all is said and done and even without it a lot of it is pretty human readable.
A casual search gives me some for 1 and 3, however 2 mostly has the sound aspect covered and not much elsehttps://www.romhacking.net/documents/525/
That said the ROM and RAM maps onhttps://datacrystal.romhacking.net/wiki/Mega_Man_2
in general has a lot of good info I would be delighted to have. There also appear to be a fair few hacks already that you might be able to compare to a stock ROM and figure out how they work, or otherwise incorporate them into your hack.