Endless Ocean and probably a Train Simulator must be the most peaceful.
Flower and Journey are mostly peaceful but they have their tense moments within the overall emotional arc.
Personally though I find the most peaceful game ever is Shadow Of The Colossus when you're not fighting. Just riding around, finding little oasis with turtles and wildlife, the sound of the surf by the ocean... outside of combat it's sublimely peaceful experience.
On that note any open world game where you can duck combat can be very peaceful, even GTA - just driving around in rain at night with ambient music is actually very peaceful. Read Dead Redemption, Skyrim, Oblivion, etc. can all be pretty peaceful and have some lovely sights.
On that note I should probably add Myst titles which offered a pretty peaceful environment to explore in most cases.