Garmin HRM-Dual, though if I were buying a new one I might go for the HRM Pro because it gives you some additional cool running dynamics stuff (how much are you bouncing up and down, how long are you in contact with the ground, etc)

A lot of wrist heart rate sensors can get "cadence lock" where the HR reported is your running or cycling cadence. For me if the watch is tighter and above the wrist bone, this happens less, but I use a chest strap for running and cycling now. (I also have a fenix 6)