The Polars are the best, but they run $100 to $120 depending on the model unless you can find them on sale.
Try to get one with a cloth chest band (not plastic). I thought the chest band would be awful, but not so much. The ones that are just a watch you have to keep touching the sensors which is hard to do in mid exercise.
Highest weight: 335 lbs, BMI 50.9
Pre-op weight: 319 lbs, BMI 48.5
Current range: 135 - 139 lbs, BMI 20.5 - 21
Goal range: 140-144,BMI 21.3 - 22 (currently trying to gain back 5 pounds)
175+ lbs lost, maintaining since February 2012