Ok, so I've had several different generations of ones through the years.
Started off as some trials for PhD with NikeFuel and JawboneUp (old version) -never got on with either and they were...not accurate.
Personally own and used two generations of Fitbit, and now have a Garmin Vivofit as it has the ability to capture and display HR from any ANT+ sensor.
It all depends what you want to do with it?
The Vivofit is the first of the new generation of activity monitors that strikes me more as an inactivity monitor - it has a quick and easy way of letting you know if you've been sedate - this is what I use it for. Recently moved back to a more desk focused job from standing 9-5 so I wanted it to make me get up and move. Which it does. Also, it has a user changeable battery and requires no charging.
I miss the vibrate alarm from my Fitbit, other than that...it was a bit basic as it had no real display. But, it was teeny tiny.