We will be visiting friends in Bethesda. Taking a flight from Buffalo. Trying to figure out which airport would be best to see our friends and/or which would be closer for them to pick us up if they can.

IAD BWI DCA...

If they can't pick us up...do any have good shuttle to Bethesda or a train to Union Station or......

Thanks..

If they're picking you up, distances aren't TOO different. But traffic can be, depending on time of day.

If they're NOT picking you up, then go to DCA. You can take the subway from airport to Bethesda, with just one switch of trains.

I think BWI is a bit further than IAD or DCA. Certainly taxi fare from BWI is higher than from IAD or DCA. Drive times can vary as much as 30-45 minutes depending upon the time of day.
Agree with rkkwan that DCA is best and it is also the least expensive alternative for public transportation since you can easily take the metro from DCA to Bethesda.
http://www.wmata.com/

We shared a shuttle from DCA with a couple that was going to Bethesda. Shuttles & taxis are easily accessible at the DCA airport (there was even a gentleman there to help us figure out which was more economical).

DCA is the closest and has the added advantage of being on the metro, as noted above. But in terms of the time it takes to drive between Bethesda and the three airports, you're probably talking about no more than about 10 minutes difference. Traffic, of course, can change that equation. If you're flying in during morning rush hour, both IAD and BWI could take longer. If you're flying in during evening rush hour, IAD and DCA would probably risk the longest delays. If not during rush hour, it probably doesn't matter. Whenever we fly out of the DC area, we usually go by whatever airport/airline has the cheapest flights; there really isn't that much difference.