Unless we are talking about a vintage NOS Hyperglide FW, it is hard to beat a new ramped HG freewheel. (The ramps improve shift quality substantially.)

Eight times out of ten, I have replaced the used FW's on bikes I've purchased because of skipping problems under heavy load, and consequently, I've purchased dozens of new FW's. (Mostly Sunrace HG 6-speeds.) In all that time, I've only had a problem with one of them (for a chain-pop noise off of one cog).

PS- be sure to replace the chain at the same time.