What’s the best reward for yourself when you completed a habit?

Depends on the habit itself.

You drank a glass of water? Probably just the appreciation of the fact that you’re hydrated.

You did 45 minutes of uninterrupted study? Maybe watch an episode of your current TV show of choice or 30 mins on a game you’re playing at the minute.

I’d say the scale of the reward should match the scale of the achievement.