If it was for deer, and only deer - and I had use just one cartridge for the rest of my life:

270 WSM would handle that.

Some of the deer are bigger than others, you can load down for reduced velocity if needed, but you can't load up for more.

Not trying to hi jack this, but also don't want to start another thread... which option would be better for 11 year as far as lower recoil? 6.5 or .243? Thanks

Look at the Howa 6.5 Grendel, plenty of round for deer to 400 yards and less recoil than the 243

In 45 years hunting deer I have used IIRC 34 different rifle calibers for deer many of which have been named already. Last year I had a little 6.5 Grendel bolt action built and that has mostly replaced the 34 year old Remington 788 in 7mm08 which I still reach for occasionally

Another one for the .25 calibers. Mine would be one of the two 25-06's I have.

As for calibers used to take a deer?
17 Rem
221 Fireball
222
223/5.56
243
25-06
6.5x55
264 Win Mag
270
280
7mm-08
7mm Rem Mag
308
30-06
300 Win Mag
358 Win

I think that's all so far

I hunted with my 7mag and 300mag for close to twenty years but the older I've gotten the more recoil sensitive I've gotten, bad neck, and today hunt with either my 6.5 CM or my 6.5X55 Swede primarily. I'm glad I have my 7mag and 300mag but boy were those two calibers a little too much for Texas sized deer. Yes they certainly work but you just don't need that much gun in Texas to take down deer. And at my age it's a lot more fun shooting my 6.5s.

.270 Win
Jack O'Connor convinced me long ago and the years and hunts have not changed my impression.

Nostalgia. I think there is a soft spot for a lot of hunters because it was a really popular deer round for many years at a time where the market wasn't so saturated. The 243 killed it off. When I think of a pure deer cartridge, I always think of the the 257 Roberts.

Ahh, yeah. It's really all subjective. I personally have a thing about the .280 Remington and I know a 7 Mag will outpace it, but I just like the .280, and for a number of different reasons. The .280 will outpace the 7x57 and 7-08, but I'd say all four of them do the same thing within certain limits. Pick your poison, choot, and be happy.

I've always thought the 270 was about as good as it gets for deer. That said, I use my 260 these days, and I consider it to be a "270 Lite". I used it on a medium sized buck about 4:30 today. I haven't seen that much of a blood trail in a long time, and I don't know what a bigger caliber could do better. Lung shot at 75 yards with a 120 gr Nosler BT. Exit the size of a lemon. I had to drag him a short distance, which was easier to do 40 years ago.

As for the 257 Roberts fans, the 260 and the 6.5 Creedmore have very similar ballistics to the 257 Roberts Ackley Improved, as long as we're talking 120 and 100 gr bullets. Just about perfect.

I use my .270 Win. and .30-06 as my go-to's. Also love my 7mm-08.