First of all, and this is probably obvious, you want balance. The ocean is cool, but too much ocean can really screw up your production until you get miniaturization late in the game. Same goes for grassland. You want your sqaures to compensate for each other, and give you a little of everything. That being said, mountains, glaciers, and tundra are pretty pointless without specials, and you should try to avoid them -- but not at all costs. Because there are two things I find more important that great overall city squares.

The first, of course, is the 4-way special.

Apparently (I learned this here), specials often but not always occur in a kind of swastika pattern, thus:

oSxxo
xxxxS
xxCxx
Sxxxx
oxxSo

where C is the city itself, X is an ordinary city square, S is a special square, and O is a square outside the city. Ideally, you want to build to get all four specials, but this isn't always wise (when, for example, the C square is a mountain or tundra). But you want to go for as many specials as possible. Note that you will often find this pattern, but there will be no special in one of the four squares; sometimes you can "find" the special by transforming the terrain (in my most recent game, grassland sitting where a special should have been turned out to be a pheasant once I transformed the land to forest).

Second, I think maybe the most important thing for my game is city spacing. I tend to build my cities as compactly as possible without overlapping; my ideal is to always to have any given city be no more than five spaces away from it's nearest neighbor, and no closer than one overlapping square. This maximizes size (because you'll build more cities) and resource use (because you'll eventually get all the good stuff) while minimizing travel time for settlers, caravans and military. Building this way means fewer cities in ideal sites, but I'll take 2 great cities, 8 so-so cities, and 2 dogs over 6 great cities anytime.

Finally, as I said, I play ToT -- but I don't know what you mean by "Landforms." That wouldn't be some kind of in crowd code, would it?

You have two extremes, 4-special cities and ICS. If you don't want to play either of these to the max, then I would suggest a happy medium of at least one special per city. Two would be better but I'd avoid trying to ensure three and four in each one. (Although, a couple of 4-special cities would be quite potent.)