Most unfun military rifles (IMHO):
AR-15. Like shooting a .22 semi-auto. I asked the owner of the thing, who also found it to be boring and was trying to sell it to me, "So, when do we start having fun with this popgun?"
Mosin-Nagant. Lousiest build quality and finish I've ever experienced in a rifle. Blow your eardrums out. Horrible groupings at mid-range. Kicked like Jackie Chan. Sporterized the thing to get some quality into it, still sucked (but it sure looked wicked). Traded it off for 2 bald flat retread tires and counted myself ahead. LOL.
SKS. Ugly. Ugly. Ugly. Bad groupings. Did I mention, UGLY!!!? Got rid of it PDQ.
Now for the MOST FUN military rifles.
1) M1 Garand. Looks great, sounds great, shoots great, living history in your hands!!!! Unfortunately, I've only been a guest shooter of them so far. Someday I hope to own one. *sigh*
2) VZ-24 (or, what the h*ll, any military Mauser). Sporterized or original, it's just drenched in quality, good looks, great performance, and general COOLNESS! Definitely the one you take home to meet the family!
3) K-31. They're so beautiful it's almost a shame to load and shoot these unparalleled works of fine military gunsmithing. The LOOKS! The SOUND! The ACCURACY! The PRECISION! Everytime I shoot mine I utter a brief prayer for the original Swiss owner as determined by his name on paper inside the buttplate: "may God bless you and keep you, Johann Reidel, for the exquisite care you gave this rifle during the years it stood ready in your Chalet to repulse Krauts, Commies, Wops, etc. Peace and Good Groupings be yours always, friend!"