After a mild Heart Attack and cardio rehab, I use a Polar F6 which is fine on bike, running and the pool (and counts cals).
I think any of the big makes are good, Polar, Sigma et al, it's just a case of choosing what functions you want.
I find, the alarm important if you are BPM limited and also the "coded" function which prevents picking-up signals from other HRM's. Other than that, just go with what looks good and displays what info you want!
Files and Data transfer stuff is all beyond me!