The GT2 pulley with 20 teeth or grooves is one of the best choices for 3d printer construction. Compared with the T2.5 or MXL timing pulley, the GT2 pulley tooth profile is anti-backlash.
Why? The rounded teeth of this tooth and belt are engaged together longer providing a smoother transition between teeth.
GT2 Aluminum Pulleys (the size you choose)
2 x M3 setscrews