it realllly reallly depends on what you're gearing for... if you're looking for higher speed my personal suggestion is a 14/39 or a 14/38 if you're looking for more power.. go like 14/42 ish... in that range.... uhh from stock gearing either way you'll notice a descent gain.. its not like durastic to the point of sh*tting yourself in disbelief.. but you know..
another very very very common gearing is 14/40.. it pulls hard, but gives you a bit more in the speed end of the deal