I've never EVER had problems with Systema gears in 4 of my guns, but I'm always willing to pay extra for guaranteed quality. All you need is Systema Flat Standard gears:
http://www.wgcshop.com/pcart/shopper...%20Gear%20Sets
If you're pushing a strong spring and don't mind a trade-off in losing a little ROF for more shots per battery, go with Systema Torque-up gears.
http://www.wgcshop.com/pcart/shopper...%20Gear%20Sets
The first batch of Guarder gears had a problem with the sector gear IIRC; I can't say for sure they've fixed it. MadMax knows more about this than I.