I can't vouch for most of the Mega Man X series, having only played the first game, but there's not really a forgettable song in that whole game. Better than Mega Man 2's soundtrack I think.
Let's see.. Final Fantasy 7 has a great soundtrack. Only good part of the game imo. I love the music to Secret of Mana too. Really anything Koji Kondo does is nice.
I think video game music is a funny thing, I can imagine the composers must be under different restraints from normal songwriters, since they are writing music that is meant to be looped infinitely, so it has to be REALLY good music for them to consider it, I would think. Just the idea that these are all actually very short songs, but can hold up to repeated listening and not grate on your nerves, I think that's what makes a good video game song.
Edit: I agree with Derek 100%, David Wise is a fucking genius. Rare's games have some of the best video game music thanks to him.
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