I should note there's one glaring omission, and that's 4Ground. Their stuff is beautiful, but as it's prepainted it's too rich for my (extremely frugal) blood.
My favorite company, by far, is Sarissa Precision, makers of the semi-famous-in-some-quarters "ToD Pods". They've got a huge and varied range in a variety of scales, extremely good (like, insanely good - 2.50 GBP flat rate worldwide good) shipping rates, and something I've only really realized after putting some other maker's models together is that their name is not just empty ad copy - it really is precision-crafted. Their stuff goes together wonderfully, fits just right, and is generally a joy to deal with. They don't really do "gothic sci-fi," and have a weird dearth of walls, but aside from that if there's something you want in 28mm they probably make it or something like it. Their per-model prices are on the high side, but you more than make up the difference in shipping, and I really can't oversell the quality. Plus they use 2mm instead of the industry-standard 3mm MDF, which may not sound like much but ends up looking a lot less chunky than anyone else (although I don't like the longevity chances of all those guardrails on the hab pods).
I have lots of their habitation pods (from the "system sci-fi" range, which also has a pretty extensive selection of gantries for your Necromunda needs):


And I got this little house from their "far east" range, which works a treat for Malifaux:

And in 15mm, the wrecked farmhouse on the left here is from Sarissa:

(The other two are from GameCraft, which makes good stuff but recently, like most American companies, has dropped out of the running for me because of shipping costs - flat $20 to start in their case.)
Next after Sarissa for me is TTCombat, the company behind that fantasy pro wrestling game, and who bought out Hawk recently. They've got ranges focused on 40K (SciFi Gothic), Infinity (SciFi Utopia), Necromunda (Industrial Hive) and more. Shipping is OK - it ended up being about 1/4 the total cost of an order for me - but quality is quite good and they have some very affordable stuff. Their 20GBP Ruin Sector in particular is a fantastic value (2 nice large-footprint modular ruined buildings, a couple shipping containers, 5 barricades and 3 low walls), and a good start for a 40K board. Since I'd already started on a more "generic science fictiony" board with the Sarissa pods, I didn't go that route, but I did get a set of their generators and 4 of their little (very nicely stacking) chemical storage tanks, the former from the Infinity-oriented range and the latter from Necromunda.


The storage tanks also came with some barrels, which I'm using for objective markers - they could work nicely in scatter terrain as well.

Finally, in a fairly distant third place is Miniature Scenery, which I got mostly because of Sarissa's previously-noted lack of sci-fi walls. They have a pretty good range of both fantasy and sci-fi, the latter being notable for having quite a few vehicle kits designed as relatively cheap alternatives to some GW stuff - Grot artillery and Ork buggies and battlewagons most notably, although they're now making some stuff pretty clearly designed with Nurgle in mind too. They also have some Malifaux-ish buildings too. Really, though, I wasn't too impressed. Their kits seem to have quite a lot of wiggle room, meaning things will to together at odd angles if you're not super careful, and some pieces are wobbly after going together. I got a couple sets of walls, and just for fun a pack of mini forklifts, crates and pallets - and the forklifts were a bit of a pain to put together, I tell you what. Still, they look OK, and they'll do the job. Shipping's a bit more expensive than TTCombat too.


I also picked up some very cool-looking Malifaux-oriented buildings from a place called LaserCraft Workshop, and while they're nicely-designed characterful kits, it's another American company and shipping is ridiculous - I managed to get some sent to my parents' house and hitchhike in a package they were sending anyway.

So those are my experiences with MDF scenery! I should also say to anyone balking at painting up scenery, it's really super easy. The only one of all these that took me more than an evening to finish was the LaserCraft Workshop houses, because the etched detail was so shallow that I ended up painting each board and shingle individually.

