When I first went fixed I started with 39/15. That was fine for a while, but eventually I went to 42/15. Mechanically its only a slight difference, but very noticeable if you ride the same hills over and over.

One other suggestion...if and when you plan on getting a singlespeed/fixed specific hub (rather than using a converted road hub, which is also fine for a singlespeed), I would suggest going with one that is threaded for a fixed gear (one that has the usual threads, plus reverse threads for the lockring). You can use a freewheel with that type of hub, but if you decide to go fixed, you will have that option without having to change hubs.