Haven't shot everything out there, but I've played with just about every category of firearm except maybe a bullpup, and I'm partial to carbines.
First one I ever got my hands on was an M1 Carbine. It shoots its own round, .30 Carbine, which looks more like a pistol cartridge than a rifle round, with its untapered case and round bullet. The M1 Carbine was a lighter rifle designed to supplement the (completely unrelated) M1 Garand during WWII. It really is a joy to shoot, historic, and nice-looking.
Next one I got to shoot was an AR-15. It's an easy shooter, with even less kick than the M1 Carbine. It almost feels like the rifle has a shock absorber in it or something.
My favorite is the AK-47, though. It kicks a bit, definitely more than the AR-15 and probably more than the M1 Carbine, too. But it puts out a pretty big, amusing fireball, and watching for the fireball is a good way to make sure you're not flinching. Also, assuming you're using the standard slant-cut muzzle device, the push is straight back, like a 1911.
Bring back Reed V. "Pie" TurkPalmer Eustachy.