GPS watch for me.
Started with a Garmin Forerunner 15, small and basic but the battery only lasted 4 hours so no good for marathon distance. Upgraded to a Forerunner 230 (good to 7+ hours), which I used with the heart rate monitor strap, then decided I wanted the built in heart rate monitor so went to a Forerunner 235.
I have pretty thin wrists so some of the bigger watches look a bit daft on me, the 235 is about the biggest i'd go.