Col. Whelen used a lot of caliber/cartridges over his life time, from small American rounds to British big bores and liked them all. He said the best two all around for America was the 257 Roberts and the 270 Win., He also saw the advantage of larger bores and with James Howe they developed the 35 and 400 Whelen cartridges. He also said you can't go wrong with the 30-06. His largest moose was taken with the 270 win.
Jack O'Connor who used the 270 win on all North American game and many world wide said elk was approaching 375 H&H territory.
Elmer Keith hated the 270 win and called it a coyote cartridge, later said if a person was a good shot the 270 win was a capable elk cartridge. He thought highly of the 45-70, he spook highly of the 35 Whelen and the 400 Whelen. I have heard the he did not have a favorite cartridge just wanted one that could throw a 300gr. bullet. But it seams that he became very fond of 33 calibers as he co-developed the 333OKH and 333OKH mag. and used them extensively.
For me if I was to just be hunting deer and hogs I would probably stay with my 270 win. But I now live were I hunt or can hunt pronghorn, deer, black bear, big horn sheep, mountain goat, elk, moose. Can't hunt now but we have grizzly bears too. So if I was going to have just one it would be my 35 Whelen, but my 400 Whelen my get the chose too.
It also drives my crazy at how many think any thing over 30 cal. is going to blow a big game animal up. A lot of the size of hole depends on the bullet used. A 375 H&H can be less destructive on a pronghorn then a 270 or 30-06 if the 375 has the right bullet in it.
As to the OP's question, he has lots of choices, looks as he is thinking of just hunting the lower 48 and maybe lower Canada, he will need to decide on what range he intends to shot and go from there, and he still has lots of good choices.
To those who say leaver guns are not accurate, they most have not used them as there are many who hunt with them all the time and are very successful, the ones I have shot very well. Leaver actions have been used for over a century and proven well.
If a person really wants a one rifle cartridge to hunt it all any where, they would get a 458 Lott and hand load. You have a good selection of bullets, you can load down to black powder 45-70 levels all the way up to full 458 Lott loads. This would allow you to tailor loads for small hogs and whitetails up to dangerous game and every thing in between.
