Shadow of the Colossus is definitely not an RPG. It’s difficult to pick what genre it is, but if I had to pick one, I would call it a “platformer.”
The game is basically 16 boss fights, so in that sense it’s an action game. But the bosses are gigantic moving statues and in order to beat them, you have to climb/jump up them. A few of them fly. One particular (my favorite) requires you to ride your horse up alongside it really fast and then jump onto its moving body.
In between these 16 encounters, you ride your horse around this eerie, quiet, ridiculously huge and beautiful overworld. It’s very serene and atmospheric and reminds me a bit of Myst. There are no minor enemies to fight, no NPCs to talk to, no towns or anything, really, except a few lizards scurrying around. It’s empty and lonely, but it works because that loneliness actually enhances the game’s atmosphere (and the bond you feel with your horsey).
SoTC was my favorite game of the last (PS2) generation. It reminds me of Halo Reach because both games are so focused, almost in a minimalist way, on atmosphere and thrilling action. There is no chaff to either game. No boring, unnecessary, repetitive encounters or sidequests that serve only to pad the “average play through time”—just a pure experience.