Depends on whats comfortable for you, and what level you are at. My set-up is as follows:
L + R: Auto-boost start
A: Boost accelerator
B: Shortfire booster
L: Regular drifter
R: Regular drifter
Down: Wide drift
I use L + R to get auto boosts every time, but then switch to A straight after since it doesnt have the same 10kmh top speed penalty that the auto-boost starter gives you, and it allows for a higher top speed in general. I use shortfire booster for B because I use Crazy Bear, which requires a lot of boosting to get it to 1900kmh, and thus the shorter the boosts the more I can fit into a run - if you are using machines with greater boost or speed deacceleration, or dont want your energy eaten up so much, use the longbooster instead. There are a multitude of drifting options that I find personally useless, although other users seem to have managed to obtain good results with them.