What Garmin Forerunner is the most accurate?

The 610 seems a little bit more accurate than the 405CX (410), which is way more accurate than the 301/305 or 201/205

They are all the same.

newest ones will be the best. 210, 410, 310xt and 610. any of the '01 and '05 models will not be nearly as good. hope this helps.

Well the 305 and 405 are exactly he same I've worn both at the same time to answer this question myself.

I have the 305, 405CX, and recently got the 610. I have had a few problems with the 610 and actually sent it back to Garmin. I loved many of the features on it, but it would not hold the charge, and the satellite readings were not accurate (pace and distance). I was disappointed because I loved the 405CX and I really thought the 610 would be a better watch, but with the problems that I had I would suggest the 405 or 410.

Why don't you ask Garmin directly whether (and how) the measurement has changed over the years?

I find my 305 plenty accurate enough for training, but I wouldn't use it to measure out a championship course. I don't think I'd use any GPS device to do that.

Note that while my 305 seems to give a reasonably accurate and reliable measure of overall distance covered, the "instantaneous pace" reading is almost useless.

I have a 405 that needs a battery replacement. It is 2 years old and the battery life is only about 2.5 hours max when using the GPS feature. With the weaker battery, my 5 mile run was more than .25 off by the time I got back to my starting point....

I got the 310. Ive run 2 marathons, and it showed the distance as 26.39 and 26.4. Thats pretty damn good Id say.

FR50/60

I once did a test in two different road races where I asked a about a dozen Forerunner wearers to report to me their readings of the course distance after the race. The group was wearing different models (201,301,205,305). The difference between the average reading for any of the models was not different from the average of any other model by more than 0.2%.

The Garmin regional product training rep stopped in the shoe store where I work a few weeks ago. Even though we don't carry Garmin he was awesome and answered a ton of questions. I personally have an older (huge rectangle shaped) 201. I hate using it as it is way to big and uncomfortable plus the satellite lock is unreliable, I mean that if I run through the shadow of a telephone pole it will lose the signal. The rep told me that the new current models are night and day better. Just like computer technology is much faster, and smaller than 5/10 years ago, so is the GPS technology that is used. He also said the very best way to use the watch is with the foot pod just incase you do lose the satellite signal. I asked if the lower models have inferior GPS technology and he said that the GPS is the same across the models, the only difference is in the features that the watch has (intervals, pacing, memory, etc...) The best thing about the current Garmin watches is their size, they actually fit like a watch and are not heavy, the 610 is no bigger than the Timex 150 lap tap watch. I'm seriously thinking about getting and actually using a new Garmin, probably a 210.