The most deadly/efficient rifle in the world today is indisputably the M16A2-A4 series and its cousin, the M4. It's ergonomic, very accurate, light, accepts a wide range of optics and other "tacticool" gear, has a wide range of possible receivers (6.5mm Grendel, ), and is the weapon of choice for Navy SEALs, Marine Force Recon, Delta, the SAS (yes the use M4s, not C7 Diemacos), and countless other organization with whom danger is their business. Yes, the early AR series had its faults, do to misconceptions and DoD failures, but they've been addressed. The direct gas impingement system, while dirty, saves space, weight, and lowers recoil, and hey, the AR family has gas piston receivers if you don't want to worry about that. Just clean your rifle, like any decent rifleman ought to be doing, and the M16/M4 family will pay off.
This isn't to undercut the AK, yes its very reliable, but it isn't very accurate, ergonomic, efficient (bullets used for targets hit) or modifiable.