Martha remains my favorite companion for being so very SENSIBLE.
And Derek Jacobi and John Simm were just stellar in their guest roles. Comparing Simm with his role in Life on Mars... Wow! He is quite an actor!
Oh, and I didn't mention it at the time, but I also quite liked the episodes "Human Nature" and "Family of Blood". Excellent work from the baddies, and, of course, they have Jessica Hynes. I realized, watching them, that she's actually only middling-attractive when she has a serious expression. It's the way her face lights up when she smiles that makes her stunning.