Mass Effect and Mass Effect 2. Even though I flat-out broke my Mass Effect 2 program (I only have a single-core PC, you see), I must say that it was without any doubt whatsoever, the very best Action RPG I've ever played.
What's neat with ME, is that Bioware ain't stoopid. As a practicer of physics, one is thrilled with the sheer amount of scientific detail used in ME. The spaceships shining blue when travelling FTL? That's Čerenkov-radiation, radiation emitted when particles travel faster than light in a medium (often other than vacuum). Faster than light travel? Well, yes - it's possible provided we can achieve negative mass! And so on. Loved the game, still do.