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
The 6.5x55 Swede is one of the most accurate calibres on the planet, low recoil makes it a joy to shoot.
However, my all time favourite is the Lee Enfield Number 1 Mk3*. Getting a good one or any Lee Enfield for that matter is a problem, many previous users have fired a crap load of ex military cordite ammunition, leading to serious erosion of the chamber throat.
One of my Lee Enfields was built at Enfield Lock in 1918 with a pristine barrel, after a bit of work I managed to get accuracy to just a tad over 1 MOA using hand loaded ammunition with 174 grain projectiles. Getting a Lee Enfield to shoot accurately can be a bit of a problem and requires a lot of patience.
I don't like the AK47, although a tremendous reliable firearm, I find it uncomfortable to shoot.
One of the best more modern rifles I have fired, has to be the British version of the FN FAL, the L1A1. Another favourite was my last issue weapon, the 9mm Parabellum Sterling SMG.
LMG,s has got to be the Mk3 303 BREN and the Gimpy.
Handguns I vote for the 45 ACP Colt 1911, 9mm Parabellum Browning Hi-Power and the 9mm Parabellum CZ75