M4a1.
an advanced weapon with good range and high power, suitable for powerful armies.
Although the action on it makes it a horrible choice for extreme conditions. Primarily very dirty, sandy conditions.
I voted M4.
[Edited by Greendaysuks136, 1/4/2010 6:20:27 PM]
Wanna know what a massively UNDER used weapon is? The H&K XM8 assault rifle. It may look like a toy because it's made primarily of weapons-grade plastic for the frame and externals, it's reliable and a good choice for any condition due to the modular nature, ease of use, ease of cleaning, and ease of assembly. Plus it's a sturdy all-around weapon.
It was also a test model and the project was canceled five years ago.