Jenny's psycho ball thing from NES Bucky O' Hare can probably contest Metal Blade. While it may not be as braindead easy to spam (moreso when it's a charged attack), once mastered, Jenny just obliterates THE ENTIRE GAME alone because the ball is a constant ripper that does damage each frame and you can make it move slowly. Sure, it might require beating two stages because konami can't into Megaman-like design philosophies and made the Blue Planet (the one where beating it, you unlock Jenny) unbeatable unless you get lil robot boy that has an arcing shot that breaks ice blocks, but once that happens the rest of the game is ez mode.
And this is without mentioning that in Bucky O'Hare you just press select and you insta-switch into different characters while in the NES Megamans you need a menu to change stuff so it's like you get to have a metal blade.tier weapon switch back and forth with other weapons in a much faster way.
TL;DR: Play NES Bucky O'Hare, it's a good game even if it's technically shorter than the first Megaman.