Any high grade potato vodka. Straight up. I usually order a couple of lemon wedges on the side and squeeze these into the martini at different intervals.

As for "best" martini, not so sure. If it's made the way I like it, then the mix is pretty much the same everywhere I go. However, for posh locations with ambience I have always enjoyed http://www.thebombayclub.com/ where I happen to be the first part of this week.

M8