This demon from hell.

While I doubt it's actually the hardest, it's so burned into my memory that I have to reference it.

The first platformer that I beat from start to finish as a kid (I think about 7 or 8) was Aladdin on the Genesis. It took months from what I recall, but I finally reached Jaffar which involves constantly running (the floor is fire so if you stop you take damage) while some force sucks you towards him, and somewhere in that mess you need to both hit him with like 18 apples (I think) and continue to collect apples up on platforms to do that.

Of course, when I finally hit him 18 or so times it looked like he died for a moment and I celebrated, but instead he changed into a snake and you have to do the whole damn thing again. I died immediately of course, I'm pretty sure I kicked the damn genesis, and it took me weeks to finally beat both forms. That was probably the greatest accomplishment of my young life at that time lol

For some reason I'm having trouble thinking of difficult final bosses otherwise... in all the RPG's it's just a matter of coming prepared, and in platformers they're usually fairly simple even in very tough games.

I'm really drawing a blank; for some reason what keeps coming to mind is actually something where the final boss was incredibly easy lol... For me, beating the 6th (final) level on the second play through of Ghosts & Goblins for the good ending was the hardest "boss", the boss being the entire level itself. Obviously not what you're asking, but I'm strangely having trouble thinking of much and as I look at my library I'm noticing that hard boss fights are mostly a thing of the past (with a few notable exceptions).