I've got the Fitbit Flex. I can track food eaten, water intake, calories burned, calories eaten, steps taken, how much sleep I have (or don't have in my case
lol) and can also do challenges against any friends and family that have a fitbit. You can log exercise too and it has a general idea of how many calories you've burned doing said exercise.
It sits in a bracelet and you wear it just like a watch. I forget I'm wearing it most of the time! I got mine on offer from Amazon in Dec 2014 for £40 (I think!)
I did look at others, but settled on the Flex in the end, no particular reason why, I just liked it