For the longest time growing up it was Baldur's Gate 2. Was such a formative gaming experience for me as a kid and several of those companions feel like reuniting with old friends.
More recently, Civ V on a medium difficulty is like Solitaire for me. You can just kind of go through the motions and enjoy building an empire.
For a different kind of relaxation-through-stimulation I also kind of find myself going back to Ninja Gaiden 2 recently. It's hard to explain why this would be comforting (and the game can also be infuriating at times) but there is just something to the combat and the constant stimulation that can take your mind off things. Especially when you go back and just replay the best parts in chapter challenge as opposed to a full campaign.