The Garmin Enduro 3 is the best GPS watch for ultra trail running, full stop. After 6 years, 4,280 activities, and 606,218m of elevation across four continents — it has never failed, never drained mid-race, and never lied about where I was.
If you run long and want one less thing to think about, buy this watch. The rest of this page is the data behind that conclusion.