My vote would be with an XCR, Tavor, or AR180B. These would give you many of the advantages of an AR (discussed to death on another thread

) including the ten round mags that most other firearms don't have. I know, I know, Mr. Rivet is no match for Messrs. Black & Decker, but that's not what we're looking at right now
The AR180B and XCR also have controls and ergonomics that seem quite similar to the AR-15 family. This is a big benefit for those of us that have put a lot of rounds through an AR.
Another valid option would be a .308 pump action or semi auto (like a Norc M305). Larger, heavier, less suited to CQB/FIBUA (/MOUT for you 'mericans), but it's also got better barrier penetration and tends to deal with longer shots better. Not what my first choice would be, but still a solid selection if something in your environment makes it more suitable than a 5.56mm platform.
Slowbalt...Since you seem pretty focused on having something that you can keep in your truck while you're on the road, can I make a suggestion? An 870 (or Norc/Dominion copy) with a barrel of 14" or 18" with your choice of stock may be a good choice. Pop a pair of ghost rings on it, a one or two shell mag extension, and maybe a side-saddle, and you've got a pretty effective piece of equipment. Remington even makes a version that comes with the 2 round extension and a Knoxx folder. With proper practice on your part, a pump operated shotgun firing slugs or 00 buckshot as the situation requires is a VERY effective system. If you're patient, you may be able to catch something like this on the EE for somewhere in the $500 price range...complete with mag extension and ghost rings.
The reason that I'm leaning towards a shotgun here is two-fold.
#1: Price. You're going to have a hard time getting your insurance to pay for a $3000 Tavor, a $1500 ACOG, and another $500 of accessories that were sitting in your $2000 1994 Chevy Shortbox when it was stolen

(I have no idea what you drive, but I'm sure you see what I'm saying here)
#2: Heat. Even if you're 100% in compliance with Canadian firearms laws, there is a chance that the County Mountie making a traffic stop isn't clear on those laws, and might take exception to the fact that you've got something that looks black and evil and military and scary in your vehicle. As long as you keep a cool head, this situation is unlikely to have any serious legal ramifications for you...but that doesn't make it any less of a pain in the @rse. You will never win an argument over a point of law with a police officer in that situation.
I'm suggesting the 870 because I happen to own one, and like the variety of aftermarket parts out there. In the words of my friend, "They're just like Lego!" You may prefer a Mossberg. I've never owned one, but their 590 and 590A1 are great value if you want an 18" barrel and ghost rings.
Oops. You asked for one selection. I gave you six. My bad.
