Team Sonic Racing is coming to Switch, PS4, and Xbox One; on which do you think it will sell the highest, and why?

Switch, quite easily.

Switch

Switch, by a good amount

Switch. That's where the kart fans are.

But, i expect a close race with the ps4.

Given how transformed sold on the Wii U, it's a reasonable expection. Though, we must remember that the Wii U version was the superior one at the time (and there was no MK8 yet i believe), but given that the Wii U had a smaller userbase, it's a reasonable expectation.
We'll see.

PS4, a family friendly kart racing game is missing from its games list. It's been hungry for one for quite a while now.

so far (at least from physical sales), Sonic Forces has sold more on the Switch than all other consoles combined. i think that alone is enough to call it.

Switch because it's a non-graphically intensive kart racer. Pretty sure Sonic games usually do better on Nintendo systems in general, though I might be wrong about that.

Sonic Forces has sold more on the Switch than all other consoles combined, Sonic + family games on Nin systems always is the king 

No, you are correct, Sonic games on Nin systems always sold the best, even more than all other systems combined