For a cheapie, the Polar A series. All you need is a big number staring at you. Everything else is nice but unnecessary. If you find you are getting committed to it (and your family hasn't filed commitment papers yet) an HRM that records and downloads into your computer is really good and a fine time waster. Being able to keep track of your time-in-zone totals week by week is a powerful training tool.
I have an older Polar 720i, but it guess there are issues with the newer Polar models, because the ones that have auto start/stop (absolutely necessary for training but not for racing) don't have altitude, and vice versa. People are liking the Garmin unit.