If this means the one rifle that means the most to me, well, that might well be my .308 Browning BLR Lightning, as it the rifle with which I first really grasped the concept of rifle shooting, in 1997. I had owned rifles earlier, but lacked a real mentor, so my rifle shooting was mediocre. I have acquired other really nice rifles afterward, but this one is special.
If this means the rifle that would be found with my body, surrounded by empty shell cases, that would likely be one or both of my Mini-14 580-series Ranch Rifles. These are my favored utility rifles, which is not the same thing as a rifle I would carry off to war. A utility rifle is, quite simply, more likely to be accessible at any given point in time.
For that matter, the long gun I carry into harm's way the most is not a rifle, but a Remington 870P pumpgun, with a Vang-comp barrel. Some idiots got to face that one just last night, when we had to calm a crowd that wanted to liberate a couple of their number from our custody. Plenty of felons and other miscreants have had the opportunity to look right UP the barrel, in the past.
Other than the barrel, which shoots a tight group with 00 buck, it has a GG&G lefty fore end with rail, a Surefire X200, and a Vang-comp extended safety button. (I may try a Magpul fore end soon, to allow for optimized placement of Surefire light(s), though I do like the slimness of the GG&G fore end.)