For Blast Corps Onion Plaza, I can't seem to find the track on Youtube but from what I hear on the midi, it's damn fun! I like ragtime. You just have a few enharmonic spelling errors. Music Theory time!
In chromatic descending passages, the notes should descend with flats, and with chromatic ascending passages, sharps.
Like so:

And it just looks a lot cleaner in general without having to constantly throw in natural signs there.
The only instances where you you descend with sharps or ascend with flats are with neighbor tones like this:

There are some exceptions of course, but this is generally the case.
Skipping the first bar, the next enharmonic spelling error I noticed was in bar 6 in the base. You spelled an E major chord with an Ab. If someone were to play the notes E Ab and B on the piano, it would be clear that that was an E major chord: E G# B, so that's how it should be notated. Always question yourself when you see something like that. It's hardly ever right.

Now the first bar, I don't really have a solid explanation at the moment, but it feels like that F natural should be an E sharp. I'll get back to you when I can figure out a way to explain that.
Other than that, some of those faster notes might be better suited as trills, but that's personal preference.
The Dr. Lunatic one looks perfect.
Mega Man 2 - Title Screen was very well done! The only mistake I could find was an enharmonic error on the 3rd to last bar of page 2 in the treble clef where that C# should be a Db. Other than that, it seems perfect.
I liked the "Insert something crazy here" remark.

For Sonic Jungle Zone, you should make some indication that those eighth notes are supposed to be swung. Second, listening to it on Youtube... it's in D major. Unless Youtube altered it or something. In anycase, you wrote yours transposed to C, which is okay if that's what you want.
Anyway, measure 3, base clef. Ascending chromatically. Should be a D#. Happens somewhere else too. And in the second to last bar base clef, same thing.
For Super Mario Land 2 - Wario's Castle, the piece is either in D minor or G dorian. Either way, you're missing that B flat. That's what those A#'s should be. Other than that, it looks fine.
Hope that was helpful.