Here's what I use 
I believe this is what most people use, except for L+R (Rocket Start Accelerate), I added that myself to always get rocket start without having to time it, then switch to holding the A button
L and R - Wide Slide. I believe this improves your drifting
A - Boost Accelerate, has slightly higher top speed
B - Boost Fire, stronger but shorter boost
I have heard that sometimes different settings are used, but I don't know exactly when or what