Best SHTF rifle implies you are limited to only one rifle. Assuming legal considerations are not an issue (you can own as many firearms as you want/afford pre-SHTF) then that also implies you are not based out of a fixed location or vehicle with enough cargo capacity to allow more than one rifle. Since ammo resupply will probably be an issue, the ammo you carry with you will likely have to last you the duration of the SHTF events.
Those conditions pretty much limit you to a .22 rimfire. You can carry a thousand rounds in a backpack without adding too much weight to your load. Specialty loads such as subsonics for use with a field expedient silencer (2 liter bottle and duct tape) or hypervelocity rounds like Stingers can be added in 50 packs for very little weight and space. Produced at a rate of 2+ Billion rounds per year it is the most likely caliber to be "found" if an ammo resupply presents itself.
As for the rifle, I would go with the Marlin 39. Very accurate and reliable, takedown so it can be concealed easily, adequate ammo capacity, and being a lever action it can be fired almost as quickly as a semi auto. A levergun is the "All American" action, it has no "Assault Weapon" stigma so if you encounter the authorities they might be less inclined to confiscate a "squirrel rifle" than say an AR15.
Range is limited, but if you are part of a family/group one or more persons could have a centerfire rifle for longer range shots. As the "designated marksman", the limited ammo supply would be less of a problem.
Of course, for short term use, one can always "acquire" the rifle and ammo supply of one's adversaries. If faced with an organized force with standardized armaments it makes sense to use their equipment as resupply is at least a possibility.
.308 bolt action, hands down. The round is powerful and accurate enough for hunting and self defense. It's common enough that you can get them, and a bolt action allows user feedback as to the suitability of the ammunition. If you need tons of firepower a 9mm uzi isn't going to get you there and neither is an AR, so for reasonable applications a handy bolt .308 is probably the best ticket. Might I suggest the Steyr Scout?
I would like to say AK because of its dependability but I think using what the military uses would be the way to go for ammo availability,rifle parts,etc. What Im getting at is there should be plenty of 5.56 and M-16s to be had. (I'm still in good w/ the local guard unit that I was in a looong time ago.
If SHTF your problem is where do you live, or plan on living? If in a urban setting a few 22 rifles (Take Downs) might be appropriate, stashed at more than one location. In the suburbs a good 40 cal. hand gun & 30-30 might do. On the road an AK or shotgun for close encounters, and 308 for distance. If in tinbuck too a 50 might be the ticket. Its your circumstances what will dictate what you need.
I'll reiterate the .308. It's a great 'sniper' round, but also good for closer quarters. It's readily available, and a NATO standard round. If you have fantasies about clearing buildings with your FA 9mm, I'm here to tell you a .308 would be a better choice (I'm discounting overpenetration here, since we're talking about SHTF scenarios.) In a close combat situation your best advantage is numbers, not pistol caliber smgs. If I could only have one rifle, it would be a .308 bolt gun. If I could have multiple I'd add some others, but pistol caliber carbines would be some of the last. I don't think anyone has the definitive truth, as circumstances vary, but I am a combat vet and my choices reflect my observations.
Darnit, I can't figure out the 'Obama' thing in yoru sig, can you resize?
No caliber is perfect for ever situation, but we're talking about the best compromise if you can only have one (unless i misunderstand the conversation). Ideally you'd have 5-6 or more firearms, one suited for every situation, but picking one involves making compromises...
DSA FN-FAL in .308, with a folding stock for transport, riding shotgun in car/trucks etc..., Every round will have to count!, Robust design, almost as reliable as the AK. Parts are in good supply. They were important to many African Nations that have had many SHTF conditions, that have lasted from days to years, if you see film clips from these nations, the FN-FAL and the AK remain!!!
This may sound funny, but I have a friend who kills squirrels with his .308 - and eats them! He handloads with a O Buck pellet over a few grains of (I think) Red Dot. It is accurate at 50 ft and makes about the same noise as a .22. Guess what I'm saying is people can get pretty creative when the chips are down. I guess if I had to go out the back door, I would probably have the M16 shorty with the .22 conversion unit and the Serbu Super Shorty in a holster. On the other hand, I may just take a .22 target pistol. Lots of people poach deer with .22's. Hopefully we never have to make that decision!