Polar. Hands down the best you can buy. They have a large variety of models based on your sport. Check out their website.

Hi,

I am using a polar rs200 since last april for running, biking and swimming (latter without the belt, of course), with complete satisfaction. I use it often as a regular wrist watch too. It also allows for the running speed monitoring upgrade with the footpod that can be bought separately. 

Of course you will want a different model if your first discipline is running, or hiking, or fitness, etc. 

btw timex watches are half the price but I never regretted spending more on the polar.

good luck,

Levente