Choosing but one leaves me in a quandary. Limiting myself to what I own (I did have quite a lot of fun with an MP-5 a time or two) it's a toss up between my Mini 14 and my m/96 Swedish Mauser. The Mini 14 is quite a bit cheaper to shoot, and, though it doesn't make me popular at the range, is tons of fun. The Swede is a bloody nice tack driver and 6.5x55 is much kinder to my unpadded frame than others I shoot (.303 Brit, 30.06, 7.62x54R). Ultimately I do love long rifles so the Swede would probably edge the Mini out. Fortunately, I don't have to make that choice.
Honorable Mention:
SKS, SVT-40, M1 Garand