Wasn't expecting Xenogears to be mentioned so soon.
As far as memorable soundtracks go... probably any of the Mega Drive Sonic games. It's very easy to get any of those songs stuck in your head. But beyond that is the fact that most of them focus primarily on melody, which probably is actually why they're so catchy. There's, as far as I can tell, a lot of game soundtracks (more recently than not, I think) where a lot of the music focuses more on chord progressions and backing.
This works well for cutscenes, where characters are talking. I don't know about anyone else, but I definitely might miss a bit of speech if I'm distracted by a particularly good piece of music. But when there's no talking, or when all the conversations are progressed by the player pressing a button, there's much more space for music to fill. One example of this done wrong is in Final Fantasy XIII.
There's a piece of music where it starts with pretty much just the normal battle song, and then after a little of that it gets a bit more fantastic. No problems there, it's quite good. Then, once it's done being a bit more fantastic (which doesn't take very long), it goes into this pathetically repetitive techno sort of thing. It wouldn't be as much of a problem if it only did this for as long as it was fantastic, but it lasts for quite much longer than the other bits.
This is disappointing (and only memorable because of how disappointing it is). If it wasn't for some of the really great pieces in that game, it would definitely be enough to make me lose hope for Masashi Hamauzu, and I definitely have lost some respect for him.
One composer who (at least usually) does focus more on melody is Yasunori Mitsuda, and my favourite game soundtrack is undoubtedly the Chrono Cross soundtrack. Unfortunately, the only game he's composed for that really seems that popular is Chrono Trigger. Dang.
TL;DR: Old Sonic game soundtracks are very memorable, but the Chrono Cross soundtrack is my favourite (so far).