Looking to buy downloadable hrm - any suggestions?

garmin 305

If "simple" is the priority, than the Polar F7, F11, and F55 could fit the bill.  These are baiscally fitness HRM and won't do distance, speed and pace in the run or bike.  All are WR to 50 meters.  I don't think any of them are very good for tracking HR in the swim.  All can connect for downloading to your computer.

You will have to step it up to a more complex watch like the RS400 or RS800 if you want to track speed and distance.

I don't believe any monitors will read your HR on the swim.

So, if you're willing to skip having your time measured during the swim, get the 305.

Otherwise, people are highly recommending the 310xt.  It's basically the 305, that's a bit smaller, waterproof, and has better battery life. 

The Sunnto 6 does IIRC.  I don't think having a HRM on the swim is too terribly helpful though.  However, I do like having a watch to keep track of my time.  I went with a Polar 625x fitted with a running pod and bike sensors.  The Garmin 305 had my interest until I started worrying about 1) getting it wet, 2) not having enough data space to record a complete IM, and 3) any time battery life is measured in hours, I'm out.

I would suggest the garmin 310.  I have the 305 and love it!  The big difference is the improved battery life, and you can wear it in the water.  (no HRM works in water)  It costs a little more than most HRM, but you get.... GPS pace, distance, HR, Bike speed, cadence, lap memory of all the that data, and you can down load all of that onto the computer. 

I'm not sure but the 310 might have an altimeter for accurate elevation change.