I voted mountain because that's my ideal for a Dwarven fortress. Practicality requires other biomes mixed with mountain, not least because the game won't allow embarking on nothing but mountains unless you use an add-on program.
In 40d my best fortress was a 3x2 fortress on a 50:50 mix of mountain and flat, slightly wooded grassland, with a brook through the flat part and a volcano in the mountain at slightly below ground level for the forest. I was able to build a fortified courtyard against the face of the mountain, pipe water in for a moat and for fortress supply, and access the magma quite close to the other workshops and only a couple of levels down. I carved a tower of bedrooms up from the entrance level, behind a large dining area, with the tombs at the top of the mountain. It was beautiful. There was also sand, which in that version came with aquifers, and even often swamps, because sand is classed as having poor drainage. In this case there were no aquifers for some reason. Two other favorites were a seashore arctic location with a volcano, and a two-level forested site with a brook and a volcano. I was able to get water from the aquifer for the arctic fortress, by using the will to desalinate the water.
In 31.x I'm not sure yet what biome I like best. The terrain isn't quite like 40d, not even counting the caverns. My most recently successful fortress has a brook that freezes only in one patch of mountain terrain, and not on either side where there is forest. It then goes over a waterfall into grassland. All of this in a 3x3. Volcanoes seem to be useless at best. Brooks which drain into ocean occasionally show up as saltwater, even if they are elevated from the saltwater swamp level for part of the map. Wells don't de-salinate anymore so a pump is required very soon in the fortress development if all of your water supply is salty. Sand isn't necessary for minor usage because you can buy it from caravans, though, so I can stay away from swamps. Because the underground farming requires irrigation, I feel that I can't afford to be without flowing freshwater. After some experimentation I have basically avoided using the caverns unless I really have to go there for necessary material (such as gems for a mood), in which case I disarm all the normal baddies in the raws and hope I can either develop military or seal off the caverns before a forgotten beast arises. Without volcanoes and the caverns, fuel is required for the furnaces and forges. I have modded the kiln to convert remains and useless body parts into charcoal. Timber is still needed for beds, barrels and bins, but plenty of timber comes in on three caravans a year. The wheel is still in motion as far as 31.x goes. Who knows what will happen after sapping is added to the game?
I don't care much for glacier sites because I've never had good luck with them. Temperate is fine with me as long as there isn't too much water to go through a freeze-thaw cycle. That really adds a lot of lag in 31.x. In 40d I like tropical moist broadleaf forest because I had modded in cacao bushes as a crop, but I have not messed with the crops in 31.x yet.