i'm running 43-15 cause thats all i could get my hands on...i got jacked on ebay for a 2 speed freewheel, sent him the money and never recieved my item. its ok though, i got a suntour freewheel and that sh*ts sick!!
i'm running 45-16...been racing for 8 years, riding that ratio for about 4...works beautifully. get a decent gate and keep your speed through the track, no worries about spinning, especially on downhill tracks (i.e. south park)