I hate the small caliber M4 and M16, but I liked the .50 "Beowulf" conversion. Unlike the .22 caliber that it normally fires, it can be converted to launch .50 cal rounds. Not near as long of a range, but enough punch to drop someone without having to be an actual kill (the .22 has always had problems against determined foes, it is very lacking in knockdown power).
I love the AA-12. A full-auto shotgun with very minimal recoil and can be fired in semi-auto without a toggle switch. Pull for 1 shot, hold to go full auto after a shot delay. Plus it can fire any 12 gauge round, including newly developed grenade rounds.
But my favorite firearm of all-time were developed in WWII. The weapon that won WWII, the semi-auto M1 Garand. The M1 is fast firing, precise at long ranges, and have plenty of knockdown power. That plus a quick reload time means you can put some lead downrange in a hurry if you need to, but the gun is more suited to picking your targets off instead of spray and pray.
Special mention to the sturmgewehr 1944 (STG44).The first assault rifle could have turned the tide back to the Nazi's favor if Hitler would have approved the project when it was first brought to him. Developed behind his back and deployed is limited amounts to the Eastern front, it had great success in combat. Mikhail Kalashnikov used the STG44 as the base when he developed the Automat Kalashnikov 1947 (AK47).
