Generally I look for resources. If you encompass 3 or 4 resources, you have a good city. The exception being if there's lots of plain desert, ice or tundra, or just not enough food to sustain a population of at least 6-8. Areas with lots of floodplains are nice for gp and towns.
Rivers are +2 health(like aqueduct) and if I am not mistaken, you can still build an aqueduct on top of that.
Forest squares get less useful later imo. I like my towns with emancipation and that commerce tech. Also, correct me if I'm wrong, but:
Grassland:
Lumbermill, forest: 2 food, 2 prod, 1 commerce (if along river)
Lumbermill, hill, forest: 1 food, 3 prod, 1 commerce (if along river)
Hill, mine: 1 food, 3 prod
Hill, wind mill, techs: 2 food, 2 prod, 1 commerce
Hill, Workshop, techs: 4 prod
Hill, workshop, techs, state property civic: 1 food, 4 prod
Watermill(along river, 1 side only I think): 2 food(3 with state property), 2 prod, 3 commerce
Town: 2 food, 1 prod, 7 commerce (8 on river)
Plains
Lumbermill, forest: 1 food, 3 prod, 1 commerce (if along river)
Lumbermill, hill, forest: 4 prod, 1 commerce (if along river)
Hill, mine: 4 prod
Hill, wind mill, techs: 1 food, 3 prod, 1 commerce
Hill, Workshop, techs: 5 prod
Hill, workshop, techs, state property civic: 5 prod
Watermill(along river, 1 side only I think): 2 food(3 with state property), 3 prod, 3 commerce
Town + tech + civics: 1 food, 2 prod, 7 commerce (8 on river)
Imo, forests with lumbermills really don't cut it compared to other options, or against hill versions.