Well, my needs change based on what fort concept I'm going for. I never just set out to build "Like, a fort" anymore. It's always on some theme like my glass pyramid fort, or my desert savage dwarves in bone armor fort, or my current mountain-y Fort of Steel(where eventually pretty much everything that can be made of steel, will be made of steel).
But there are a few things I nearly always look for:
-Stream. It can be a brook, a river, the ocean, whatever, I just like to know that I can't run out of water.
-No Aquifer. I'm still wussing out on learning how to deal with them.
-A single column, near the middle, that goes straight past the caverns to the magma sea, allowing me to easily dig down past the caverns and get a magma smelting industry going. Lately I've been relaxing on this one as I am getting enough experience with the caverns to realize it isn't floor-to-ceiling GCS and FBs.
-Steel making materials. Magnetite is preferred, for its clusters.
-Clay and Sand.
-Only 1-2 layers of soil. Deep and Very Deep soil isn't a bad thing, really, but I get annoyed at it still. One or two layers is plenty, then we need to be getting to the rocks, already!
-Savage Biome. This one of my huge deal breakers. I almost never embark on a biome of Wilderness or lower. I want to be able to see(and die to, and eventually capture and tame) giant beasties.
-Bauxite. So pretty...