I don't like anything smaller than a 243 for deer. Past that, it just depends on where you're hunting. Open plains, I like a flat shooter. Anything from 24-33 cal with fit that bill nicely. Heavy brush, I like a 375 H&H with a 270SP. I never miss my mark, I don't have to pass on a shot and an instant blood trail with no loss of meat. You could shoot through a small tree if need be. If it had to be one, 308 would be a good choice. I've taken more deer with a 308 than anything else. And hunted with it far longer than anything else. I like a 150gr W/W Power Point for open plains and a 180gr of brush in that case
I prefer/use a 30-06, or 308, maybe a 7MM-08 - I don't shoot game past 300 yards. I used a 7 PRC last year and it was just a bit much. I know there is no such thing as too dead, but a high shoulder shot on an eight point cost me both front shoulders and both backstraps.
I think a 280 AI would be perfect. Very hard to beat 7mm ballistics/terminal performance.
AR-15 with 16inch, 1x9 or 1x7 twist using a Eotech with 3x magnifier. Running a 60 grn Hornady TAP ammo. I've dropped everything from deer to 350 plus pound hogs with it, all 1 shot, 1 kill.
I have a Mauser action 257 Robert with 18 inch barrel Timney trigger and 2 1/2 power weaver scope weighs a little less than 5 pounds. Very accurate and one shot is all it takes.