When I was eleven years old, I bought a bolt-action .243 Musketeer from the original Gander Mountain storefront in downtown Wilmot Wisconsin. I also bought reloading equipment and supplies for it. Actually, my dad purchased the stuff but he used the money I had earned and saved for it.
I became very familiar with the gun by reloading for it, shooting off the bench with it, and using it for varmints. I started with lighter loads and worked up to max loads.The more I shot it, the better I became. I had the same comfortable feeling shooting it that I had with my .22 rifles, and I became deadly accurate and confident with it.
By the time I was old enough to hunt with it, one-shot kills were the rule, not the exception. The .243 cartridge is absolutely deadly on whitetail, and I do not use any other gun on them, except in shotgun-only areas. I still remember reading an article in Field & Stream which stated that the .243 was the gun of choice used by back-up gunners on lion and tiger safaris. The article speculated that the combination of bullet weight, speed, and rifling twist made it the ideal cartridge for medium-sized game - with the right bullet/velocity combination the bullet delivers all of it's energy into the critter without passing through. Unlike other higher-velocity, heavier-bullet-weight rounds which don't deliver the same shock when they pass through.
I know that there are other valid opinions on other rifle cartridges, and I respect them. But for me, the combination of good bullet performance and the comfortable feeling I have with bullet placement puts the .243 at the top of the whitetail list.