I've been playing through Metroid Prime 2. It's been pretty good, though keeping track of both the light and dark worlds is a pain at times. And now I've unlocked all the power ups I need to do a treasure hunt across the whole map for 9 more keys!
I played through Metroid Prime 1 a couple of years back, and I'm not sure if it's rose-tinted glasses but I seem to remember enjoying the map and bosses in that one slightly more.