SiegeTek make the strongest AEG gears on the planet. 
This is the latest generation (Gen 4) of Chromoly gears from their factory, case hardened for extreme durability. 
This full set comes with the Sector, Spur and Bevel gears.
The Sector features an enhanced cutoff cam for snappier cutoff lever activation (and fewer semi-auto lockups) and is compatible with all mechanical and optical cycle detection systems. 
The enlarged tappet cam allows for optimal feeding at all rates of fire without the need for an additional delayer chip.
The Spur is as solid as ever and will work with most standard ratio sectors and bevels. 
The Bevel gear now comes with an enhanced anti-rotation ring with a higher number of steps for the ARL to catch on as well as smoother tooth geometry for quieter operation.
 
Fully CNC machined and expertly finished, these gears will last for years to come in virtually any setup you could build. 
For absolute strength and reliability, nothing else compares.