I've been using the Shimano MX30 that came with my bike, it done 2000 miles during the winter and is good for the same again this winter. Every 700-800 miles I remove the freewheel then loosen but not remove the front cover then give the entire assembly a good swirl around in a tub of turps, then I blow it dry with compressed air and drop it into a bath of SAE-30W engine oil and make sure the oil goes into everything. It is then removed from the oil and the excess allowed to drain before tightening the front cover and refitting to the hub. It is smoother now than the day it was made.
Also keep an eye on
ebay for vintage Made in England freewheels, the quality of them are far superior to modern items.
Just had a quick look on ebay and there are also some nice vintage Italian freewheels, but they are pricey