As far as using a single cog, I usually see folks go with a 16 or 17.
If you're consider a freewheel.. I wouldn't buy a current production freewheel, I'd buy something produced back during that period when the freewheel was still king. My favorites are the Sachs (SRAM) ARIS (6/7/8) and the Dura-Ace MF-7400 (6/7/8).