I own a lot of rifles, big and small. The 243 is a fine short range deer rifle or coyote gun. Yes i know they will kill others. A 270 is a step up but again will only launch max a 150 gr projectile. It comes down more than anything on what you intend to hunt, where, and if this a "all around" gun or if you see yourself adding others later. If i was to limit myself to only one it would be a good bolt action with a good scope in any of the following....30-06, 7 mag, or 300 win. Any of those three will take anything that walks in North America. If you mostly hunt smaller game add some smaller calibers, moose or large bears a few larger...notably the 338 Win.
i have owned a 338 lapua, IMO, a long range rig for the reloaders. Much the same can be said about the Ultra Mag family, and the Weatherbys. Further all of these calibers are expensive to shoot, which makes it hard to become a good shot. Go to your Walmart, common calibers are cheap to shoot. Get good, buy a box of expensive premiums to hunt with, and never look back.
Bryan