Lots of times I'll build in forests, or on hills or even mountains. Not EVERY city, but some. They may not grow as fast initially, but they will later (if there's enough food squares around it that is), and they'll have a good defense.
Think: a vet mech infantry (9 defense), in a city on a mountain (9*3=27), with walls (27*3=81)--81 defense (or still 27 against howitzers)!!

Lots of times "mountain cities" become my capital (I move my palace there). 54 with SAM against air or with coastal fortress against sea bombardment, too.
Lots of times "forest" cities are incidental--if the square in the middle of four specials happens to be forest, I will still definitely build there. Forests aren't too bad--slower growth initially (I may have to build two warriors, or a warrior and a barracks, before I can build a settler), but more shields initially too--they often are my early "barracks" cities.
Of course, strategic location is important too--any "isthmus" location is prime real estate, even if it will partly overlap another ideal location (a "4 specials" location). If it overlaps a "4 specials" city, I will try to control its growth so that the 4 specials city maxes potential, and let the isthmus city grow in the remaining "free" squares.
Oh, and for coastal cities, whales are better than fish (IMHO), and so as is often the case, if I must choose between locating a city near a whale or near a fish (i.e. it can't be both, and if I built two cities to take advantage of both they'd be too close), I'll always go with the whale.
I also like building cities on single-square islands (after I have harbors, of course)--virtually unconquerable until the enemy gets amphib warfare, and LOTS of trade when they get big....