Prior to Mario Kart VIII, always 50cc. With Mario Kart VIII and the fact that trophies carry down, 100cc. Figured I would go ahead and realize how miserable I actually am at racing as I've always been a Battle Mode fan at heart. Seriously, if I can't get Double Decker or Block Fort redone so that I can ride on the walls and ceilings at some point I'll be very disappointed.
There's incentive to skip to 150cc I guess. I'm like most people though, ease yourself in with 50cc, and work up.
What about Stars do they carry down? I mean I've been slaughtering the courses on 50cc so far and 3-starred everything. If I have to play on the actual difficulty to get the stars anyway, it's not a complete waste of time. Also you get more coins for unlocking stuff...
Someone told me the stars do not carry down, regardless, I always start at 50cc. It really doesn't take much time to go though all 16 tracks plus I like being able to learn all the nuisances of each track.