I live in rolling terrain in what the local real estate types have taken to calling The Lakelands of South Carolina. When I rode ss mountain bikes on the area's trails (they're a lot of fun!) I ran 34x18 (edited for accuracy!), yielding around a 48-in gear on the 2.1-in Panaracer Fire tires I used to use.

For fixed-gear I've been primarily running around 70-71 gear inches on the road since 1998 or so. It was 48x18 on my old Bianchi Pista, 45x17 on my beat-but-sweet old Gitane TdF, and 42x16 on the Peugeot PR-10, Falcon San Remo, '71 Raleigh Competition and the custom Mercian. When I started adding fire roads to the equation I fitted a larger cog on the other side of the hub, typically around 63 inches. I think I rode just about every dirt road in this county and a bunch of them in neighboring counties on a 42x18 on 28mm tires.

My current all-road mad scientist bike is a 50-year-old Raleigh Competition with 35mm tires, a Surly Dingle fixed cog and a White Industries Dos Eno freewheel on the other. Combined with 42/44T chainrings, I get 70-in pavement and 60-in gravel fixed gear options, or I can flip the wheel and have a 60-in general noodling and a 52-in mild singletrack option for coasting.