Totally depends on what you want to do, and what your budget is. I am assuming you mean you want a gps for running, not directing you in a car or when walking. The most basic model (of the current models) is the Forerunner 110, and they add more features as you go to the 210 and 410. If you want multisport, including swimming then it's the 310 or 610 really (though you can use the others on a bike as they will let you do speed in terms of mins/mile (running pace) or miles per hour (bike speed) (you could do either in metric if you want.
Only you can work out what features you would like/can afford. I suggest you look on the garmin site at the comparisons between different models. dc rainmaker (google him) does very in depths reviews, and actually uses them properly to find out what they can do rather than just read the manufacturers blurb ;ike many "reviews" seem to do.