I still need to replay MM5 and 6 yet, but at least for now this one is the best between the 8-bit (counting 9 and 10). It's the first to make the player really explore, and although we have 4 extra stages like on MM3, this time they're not just harder versions of already completed stages with 2 bosses each. Now it makes sense to play those 4 stages and don't look like a waste of time
The two secret weapons are also a nice addition, alongside the charged buster that would appear on various games from now on.

I easily reccomend it