I have a Withings brand watch. Seems very accurate for afib diagnosis. I generally use it to confirm if I am having an episode. I recommend it. It also does other health measures. I also have one of their special scales and a blood pressure sleeve.

For afib non watch I think a small Kardia device is best. Covers a number of other arrhythmias as well. Under $100. It is small and easy to use but not quite as convenient as a watch. I think it is very accurate.