My favorite root beer at Ellis Island in Las Vegas. Really enjoyed it with their steak special. Couldn't get enough of it.
A&W from a tap (versus a can) would probably rate second.

I don't think you can fairly compare draught versus stuff from a bottle/can.

We had a bit of a root beer tasting (of bottles and cans) at my friends place a few months back. Personally, my favorite there was Henry Weinhard's. It's somewhat tough to get in Vancouver but I think a local grocery chain is starting to carry it. I used to have to bring a few case back during trips to Seattle. A&W and Boylan's Birch Beer were a tie for second for me. I like A&W's caramelly sweetness while Boylan's sugar cane sweetness was interesting in a good but different kind of way. Thomas Kemper and Hires are ok. Virgil's didn't do anything for me.