There are often two answers to this question.
Best case scenario: "The rifle I have with me."
The worst case scenario: "The rifle I left behind."
One of the nicest places I've ever hunted is a ranch that the inlaws own in N. Uvalde County, TX. Lots of scrub oak, juniper, and prickly pear. Plenty of deer in the Nueces River valley there, and it's a 5-deer county. First time I went out there, I stalked a fair amount with my iron-sighted Springfield, as I thought all the shots were under a hundred yards. The last morning there, I climbed up a steep hill/small mountain that overlooked the ranch, and saw that I could see the whole of the ranch, into just about every clearing. From my sit, I watched a very nice 8 point buck step out at 250-300 yards and stand in the middle of a clearing, browsing. I grabbed my binoculars and glassed him. As I started to pass them to my fiancee, I noticed something in a clearing beyond that buck. At 400+ yards it's hard to count tines, but that rack on the 2nd buck
had to be in excess of 10 points, and it was 'wayy beyond the tips of his ears.
I kind of laughed/hiccupped, and handed the binocs to my future wife. She asked what was wrong. I explained that, with the rough country I'd have to cross and the density of the cover around the deer, I'd never find them on a stalk to them. Worse, that morning I'd made a decision of which rifle to bring with me, and I'd brought the iron-sighted (and rough-bored) Springfield '06 with me again... and left my .300 Win Mag Sendero behind.
With the 180 g loads I had put together for the Sendero, I could honestly claim 4 inches at 400 yards. Perhaps less with no wind (it's a 3/4 MOA rig at 100), and this was a windless morning. With the Springfield, I was proud and pleased to EVER break 3" at 100 yards. At 400 yards, that would be a 12" group... IF my brass bead-on-post front sight didn't subtend the buck's chest (and it did).
Nice compromise rifles are a joy, though: I've done some good hunting, near and far, with a too-heavy Ruger M77 semi-varminter-barrelled .257 Roberts (that I would have taken that shot with), and plan to do some similar hunting with a .243 M70 Featherweight (Pre-64, first year production for the caliber, and as classic as they come. You are invited to eat your hearts out.) that I MIGHT have taken that shot with. The latter is also handy enough to stalk with, too.