M107A1
I know there are some cash flow and weight issues with it at $20K and 30 lbs, but to me the suppressed M107A1 is a work of art. Clearly a significant amount of engineering in energy management and design went into this rifle.
It also fires a broad range of rounds from ball, to hollowpoint, AP, API, APIT and APEX. Few other weapons have that capability, much less the range.
Besides, what other man portable weapon has 100,000 ft lbs of energy in a single magazine?