I have a crummy computer, so I prefer flat 3x3 sites.
Features I search for are:
Underground River (permanent water supply and silk, maybe tower caps)
Adamantium (elite troops. i prefer a military defense, minimal traps)
Iron (same as above, for iron/steel gear)
Marble (sounds better to have a marble floor than limestone or chalk, and its a flux)
Heavily forested surface (beds and charcoal for steelmaking)
Magma (for obsidian and steelmaking, along with precious metals for furniture)
Sand (preferably two layers, for easy storage rooms and farming. less excess rock)
This means I usually search around the edges where mountains meet forests looking for a place with Sand, Marble, and some type of sedimentary layer for iron. Then I go in an use reveal to see whether there is addy and a cave river within a 3x3 area.
Best I've found so far is 3x4 (could do 3x3 but you only get the very end of the river, before it dumps into a chasm) with sand, iron, marble, and addy. There is enough magma in the addy pit for 10 workshops, but it'd turn to obsidian with temp on, could still fuel 8 without hardening, if I did some clever pumping and channeling. Of course that means fighting the demons before getting my steel operation running. There isn't as much wood as i'd like, enough for beds but not enough for a proper steel industry, though coal deposits should make up for it. It isn't flat, but there aren't any big cliffs.
I don't think I'm going to bother building here though until I replace my PC. I tried adjuststart to give myself 100 dwarves and my FPS goes to hell as soon as they have jobs, even on a flat, empty 3x3.