42/16 or 17, depending on my legs. This is fixed, road bike. I want to upgrade my chainring though and I'm having a hard time finding a 42T, 144BCD ring so may go larger and get a new sprocket or two to match. Terrain is flat-ish and I'll cruise just under 20mph with this. I've pounded up some hills, it took some effort but they aren't that long around here. Downhills aren't so long either so while math says I've hit 140-ish cadence, it wasn't for so long either.

YMMV though. If you have a multi-speed bike, you can experiment by not shifting and find a single gear ratio that works for you for now and then go with something close to that.
I'll second heading to the fixed/single speed forum, but gear ratio will depend a lot on your legs and we can't help there.