Zorua, The Garrus mission? The best mission, so far. Apart from maybe slow motion orbital striking a Reaper.
I have 4 saves, each with everyone surviving, and different options just to see how it helps in the final fight. I am at least completing ME3 5 times.
I chose to have a relationship with Tali, just because no-one else was and now she is my favourite character. The stuff that went on, on Rannoch between Shepard, Tali and Legion was the stuff of Shakespeare.