What in your opinion is the best hunting calibre for North American Game? Why?

Besides the .308


*NOT rifle, but calibre*

I like the 7mm Rem Mag.  Flat shooting cartridge with good power.  A bit less recoil than a .300 mag.  Wide range of bullet selection from 100gr - 175gr. Capable of taking all game from long range varmints to well placed shots on big game.  I am also comfortable shooting it, that is what is most important.  

Depends on the game. I have always used a .270 win. I have a .30-06 and it shoots well.

The .270 is a good flat shooting round. However, the .270 is not reccomended by most to hunt bear and moose with, but I have had no complications. you just have to place your shot.

If I was going to buy another rifle in a hunting caliber. It would probably be a larger caliber like a .30-.378 wetherby. They only drop about 26 inches at 500 yards, not bad at all considering the 30 caliber bullet it's throwing.

For a good hunting rifle that will do it all and is a faily popular round I would get a 7mm remmington or a .300 win. mag. A .30-06 is a very readily avaliable round and although it has a fall off at long ranges it will do the trick aswell.

People would like to say the the new remmington ultra mags are so much bettter, but I would disagree. Not only are you spending a bunch of money to shoot those calibers, there is only a slight improvement in energy. The animal your killing will not know the difference between a .300 rem. ultra mag. and a .300 win. mag.

I would simply go buy a ballistics table for factory loaded rounds and compare those then make a desision. Factory loads are not as hot as they can be, but they will give you a very good idea at what you are looking at.

I neglected to mention the new .300 WSM.  It looks like a good round.  I have not shot it yet, but it will be interesting to see how popular it becomes.  .300 mag performance in a short action, sounds like a good sniper round.  Just needs to be available in more platforms.  

8" 200lb howitzer.
Whatever you hunt stays hunted.
It is kind of ruff on trophys though.

If not a 308 try 3006 it will drop anything in north america.wide range of bullet weights if you reload you have more combinations.300 win mag would be good also but has more recoil and would probaly devestate any small game at close ranges.

.280 Remington.  7mm on a '06 case.

Slight advantage in down range ballistics over the .270 Win and velocities approaching the 7mm mag with out the larger charges of powder(I get 3000fps with 140gr. bullets easily).  Lots of bullet weights and styles to choose from and plenty powerful enough for just about anything except the big bears.  I have one in M700 that literally drives tacks at 100yds.  It's been named and renamed a few times over the years.  7mm Express, 7mm-06, .280 Rem.  Some argue that if it would have been developed before the .270, it would hold that cartridges place in the annals of gun lore but IIRC it came along many years afterwards.  IMO it's the best all around caliber out there.

I like the 30-06.
1) Practice ammo is cheap
2) It will take anything on the NA continent
3) If you run out, the ammo is easy to find in the boonies.
I currently shoot a M1A Scout, but most of my Elk have fallen to a Springfield Sporter 30.06 and a Ruger 30.06.
Just a traditionalist.

If you just want one rifle, the 30-06 is hard to beat.  Accurate, abundant power, good trajectory, most popular hunting round by far, ammunition available everywhere, more avaible loads than any other round, fairly cheap to shoot, moderate recoil, excellent for everything from pronghorn to grizzly, etc.

My second choice would be one of the 300 mags.  They increase your range by 30-60 yards, but there are substantial trade-offs as far as recoil, ammo expense, and magazine capacity.

The 270, 7mag and the like are good calibers, but I think they are a little small for big bull elk or big bears.  You might be wise to get a really big gun (338, 375) for the rare occassion when you get to hunt such game.

I just traded my 300 win mag for a 280 remington.  I haven't shot it yet, but will this weekend.  The 300 win mag was too much gun for deer and antelope.  Shot a small mule deer with that once and made a big freakin mess.  Got the 280 for everyhing except african big game and brown bear.  I'll get a 375 H&H if I ever go for any of that.  Check out the ballistics: fast, flat shooting, and plenty of power. Never heard of anyone who tried it and didn't like it.  The 300 win mag was nice but after shooting 2 boxes through that thing in a prone position it just ceased to be fun.  

6.5mm either the swedish or 260 remington.Big enough to do the job with any resonable shot.Light recoil for beginners or timid people.With a good 160gr bullet will penetrate like no tommorow.Not the flatest shooting or most powerful round but will do the job.If I was offered a free bear or moose hunt and had to use the above caliburs Id take it in a heart beat.I wouldn't consider it a stunt but would pick my shots with extra care.

You didn't state what the game will be, so I'll guess that you want this caliber to be extremely versatile.  Short of taking dangerous game (the bite-back variety) and something that can be shot at the bench and in hunting positions without the gun biting you either, I recommend the 7MM Remington Magnum.
Why?  Because it can shoot 100-195-gr. bullets. That can be for varmints to moose.  Barnes Original 195-gr. bullets are the heaviest in the caliber and have a sectional density of .345.  Really good for penetration on big game.
The varmint weight bullets in the 100-120gr. range are numerous.
The 130-175-gr. bullets are equally plentiful.
There was an article written somewhere about the author trying to figure out what the "best" caliber was and when he consulted with a professor, the prof. told him he'd forgotten about the WIND.  The 7mm bullets are superior in regard to flight characterisitics, i.e., wind bucking ability; they fly very well.

So, you have a good selection of bullet weights; the most common of the Magnum calibers; excellent power and range with accuracy; and the gun/cartridge won't kill you to shoot it.

Hey, I love mine!  I've got 2.

The truth is that there are several calibers that will do what you want them to do.  Kill the animal quickly and cleanly.  Humanely if you wish to call it that.  
If I were to choose another rifle now, I'd look at the .280 Remington caliber, too.  I like the .284/7MM caliber. I do own others, but, so we don't confuse the "one caliber" question line, I'll say that the 7MM Remington Magnum would be MY choice.  
I simply love the way the cartridge feels when it goes BOOM!!!
That is my preference and reasoning.