At the moment I've been finishing the Mass Effect 2 loyalty missions. It just seems like a really well made game with great plot and good action sequences. I love the way that battles just happen spontaneously and you really feel the tension and atmosphere change as soon as a battle happens. It's a really good transition from the tranquil observation of alien worlds into the real-time battles.
I love several things such as using different skills, choosing the appropriate dialogue and experimenting with different teams and weapons.
I've also been playing the Mass Effect 3 Demo. Here's some gameplay: