If you are running a freewheel, then get the BEST with this fully re-buildable Profile Racing Elite freewheel with 140 points of engagement. Arguably THE BEST freewheel you can buy, see video below to see what goes into these.  

140 points of engagement
Two Fully Sealed Bearings
Hardened CrMo Outer Cog Driver
Titanium Inner Body Driver
Easily Serviced and Completely Re-buildable
3/32" teeth (can be run with 3/32", 9 speed and 1/8" chains)
Available in 16t to 22t.

Common ratios are:
20" bikes = 44/16
24" bikes = 41/18 or 39/17

Made in the USA!

Proprietary Removal Tool Sold Separately