I've tried a few different ones. I keep going back to "Star Trek Watch" since it seems to use the least battery of the ones I've tried.
I can't say it's the "best", but it's the best of what I've used. I can get about 36 hours on normal usage (Auto-3g, Wifi on, no auto HR, No NFC, connected to phone with BT). I get frequent notifications for work email, during work hours, and I use S-voice to send messages frequently.