I'll throw my hat in with the M14/M1A crowd. When you consider mechanical reliability, intrinsic accuracy, long-term durability, range, power, ergonomics, balance, the quality of the sights and trigger, and all the other factors that go into making a good rifle, the M14 has to rank pretty high up there. I've shot AR-15s and M16s. Stoner's rifle is versatile, accurate, and generally reliable. But it never came across as solid or as robust as the M14 to me. In terms of general shooting characteristics, no other rifle is as pleasant for me as the M14. It it truly a rifleman's rifle.
Next I would have to put the Kalashnikov, then the FAL, then the AR-15.
Not only is the Ruger Mini not the finest rifle ever, but it isn't even the finest poodle shooter. Heck, far from being the finest, it is almost the worst. I would imagine the engineers at Ruger had to try really hard to change a rifle known for its accuracy and reliability into that bastard abomination of a rifle known for its lack of both, but they managed. There are a lot of fine poodle shooters out there--everything from time tested classics like the SIG 556 and the AUG that have made it to the American commercial market only relatively recently--to newer designs like the XCR and the FS2000. Of course there is a plethora of fine ARs, as well as various more obscure designs as well like the Daewoo K2, Galil and other 5.56mm and 5.45mm AK-clones, and the AR-18 (which actually shouldn't be that obscure considering its simple short-stroke gas piston operation has been copied in many other designs). The Mini-14 is not on this list and indeed, any of these choices are almost guaranteed to be more accurate and more reliable than Ruger's underwhelming little carbine. The only reason I can think of that someone might consider the Mini-14 over any of these designs barring being obviously brain-dead is economic and even then, an SKS can be had for hundreds less and aside from being more accurate and reliable is easier to find parts and accessories for, and is a lot more durable than the Ruger.