https://www.youtube.com/watch?v=LKOfuOoCzBM – This one's a bit of a stretch (major-key melody swapped for minor-key), but I can see what they're going for, and it's got good execution.
[Wario's Woods, Kirby's Dream Land 2]
https://www.youtube.com/watch?v=BetW9dvj8Po – Wow! This person did their homework in figuring out how to fit this together.
[Cave Story, Legacy of Lunatic Kingdom]
https://www.youtube.com/watch?v=yGZr98GEs0U – Short, simple, but satisfyingly solid.
[Portal 2, Portal]
http://youtube.com/watch?v=Pv8qfHjQde4 – The arrangement has a few awkward spots, but still: about time. They should've done this one years ago.
[Final Fantasy VI, Umineko When They Cry]
https://www.youtube.com/watch?v=-_I8zpgRi_o – The only reason this ISN'T in the actual game is because of copyright. Absolutely certain.
[Undertale, ?????????????????]
https://www.youtube.com/watch?v=LrjlJ_dR-tQ – Well, uh, that's one way to improve this track.
[Sonic the Hedgehog, Homestuck]
https://www.youtube.com/watch?v=qqQuqqcL_-U – Considering how both of these source tracks are vastly overused, this combination is somehow, miraculously, a genius fit.
[Deltarune, Mozart]
https://www.youtube.com/watch?v=LbVqRicmfwM – Another old and very satisfying one I forgot about.
[Super Mario Land, Super Mario Bros.]
https://gilvasunner.bandcamp.com/track/it-was-only-through-the-siiva-wiki-that-i-noticed-a-real-boy-is-supposed-to-have-an-exclamation-mark-in-it – Super short incidental tracks are always a risk for this account, but this one's probably the best you could do with this particular track.
[Deltarune, Super Mario 64]
https://www.youtube.com/watch?v=BJZ4fLviAfA – This one's maybe a little too fluid with all 3 tracks' melodies, but it's clever anyway.
[Mega Man 2, ChoRenSha 68K]