My best MvM runs on scout have been on Decoy (which, honestly, seems like the best map for non-engies of any kind – more on that in a bit). The chokepoint right at the start seems to be an ideal spot for a scout to attack first – close quarters (good for scattergun), large amounts of enemies (also good for scattergun), and small area for cash drops (makes magnet powers easier).
Little Engineer-specific advice/analysis:
Like I said earlier up, I find that Decoy is the hardest Engineer map. You still do need one, but the best spot to set up, IMO (the roof of the building right outside the chokepoint), is a bit too far from the front to help initially, and none of the closer spots to set up have the same amount of protective power that that building does.
Coaltown is probably one of the best engie maps: I’ve seen 3 different tactically sound spots for building setups: The two roofs right outside the chokepoint and the alley directly to the player’s left of the spawn. There’s also the building with only one door right after the chokepoint, but I would only use that if there’s a second engineer; I haven’t found it to be very useful running a single engie setup.
Mannworks is right in the middle between the two. The ideal spot for setup (The building with platforms on both sides, right before the rock field, that overlooks both paths) is a bit far from the spawn, but it’s also an excellent spot to have everyone else set up anyways in my experience. The two platforms have a great line of sight over the paths, and give your turret plenty of in-range time to attack, and there are great places on both of them to hide the dispenser. A tele exit also does well inside the building, and is actually the best for keeping it against a sentry buster out of all the maps (On the other maps, you can really only hide it near the dispenser so it’s still close and defended, but inside the building, oddly enough, is just out of the buster’s range, assuming you put the sentry on the right stairs/a bit out from the sandbags [I think that’s what they are] on the left). I would give it best engie map if the building was closer to the spawns.
I find that the Engineer is actually one of my best classes in Mann vs. Machine. It’s really weird, because in Multi I’m absolutely horrid at Engie. Same with Scout, anymore I only use him on CTF to try to just go for a speed & dodge approach / I’m desperate. I feel I got lucky with Pyro, as I’m good with him anyways and he’s an extremely useful class in MvM. I’m sad that medic isn’t too good, as he’s one of my other faves.