The Diemaco C8 used by the Canadian forces and the Flam Thrower combined would be one of the most effective weapons. An assualt rifle mixed with a flame thrower for ultimate carnage and mid range combat. Plus with this weapon you can put down suppresive fire and burn out the enemy from buildings. (Or in cover for that matter)
The best combination is when you are a good shooter. I think no one of us had shot with a sniper rifle or with a rifle, or with a machine gun. I had. And I think the best is when you have a good weapon (witch has oiled, and cleared well) and a good shooting skill.
M16 with the underbarrel shotgun that is available (I believe it's the KAC Masterkey). The M16 offers you long range accuracy and killing ability while the Masterkey offers close range killing power and the ability to get through locked doors and such. Explosives such as grenades can not be used in close quarters for obvious reasons and modern combat tends to involve a lot more CQC. The other issue of portability comes in here, with the overall weapon weighing around 14 pounds, quite heavy except when compaired to other weapons.