Although I can't say I'm THAT seasoned, since my HUD is more obscure, I still have a lot for MvM.
hudmannvsmachinestatus.res - VERY IMPORTANT. Is the 'hudlayout.res' for MvM, use this file for definitions instead. This goes hand in hand with mvmscoreboard.res.
huditemeffectmeter_heavy.res - Edits the Heavy's rage bar. Two Cities update made this a copypaste with the Medic's energy bar.
huditemeffectmeter_sapper.res - Edits the Spy's sapper bar.
mvmcreditsubpanel.res - These two files are the second part of the scoreboard. http://i.imgur.com/adghM7y.jpg Here is a picture that I took with the scoreboard. I think everything else is outdated, but it's the second one down.
As you can see, the "1" and "4" are the bonus money waves.
wavecompletesummarypanel.res - Self explanatory.
wavestatuspanel.res - Changes the "WAVE 1/7" or "2/6" etc with waves. Also affects the SUPPORT panel. If you change the SUPPORT word, hud_reloadscheme doesn't show it unless you restart TF2.
enemycountpanel.res - Hand in hand with WaveStatusPanel.res, it's the actual numbers. Can change fonts and critical marker.
huditemeffectmeter_killstreak.res - Killstreak bar. Putting this here since a surprising amount of HUDs don't yet have this made.
Border tweaks: A lot of huds nowadays use boxes around ammo and health. I do, the default hud does. (Obviously, some huds still go for a minimalist style, which I'm fully fine with.)
Use "border" "BORDERHERE" in a certain panel, whichever you want it to be.
You can edit borders in clientscheme.res.
However, my minmode hud makes these boxes disappear with this little tweak with borders.
"draw_corner_width" "13371337" // more publicity for any non-hud editors looking at the hud's code
"draw_corner_height" "90019001" // who cares that this certain meme is around 7 years old
Hudlayout.res building tweak:
Thanks to something I said with the 'tall' value, omni found a nice tweak that lets the ypos of the buildings on Engie look fine on the spectator/deathcam. Doesn't work with MvM, but oh well.
Remember while using the tweak, hudlayout.res definition should have ypos on 0.
Also works with Spy's sapper. It IS a 'building', after all.
There are many ways to make a HUD, but I haven't found one that hasn't done one of 3 things with the ammo. Be careful, some HUD's fall into the Box-Squeezed problem.
Yes, it's possible to combine Box with Top-Down, but the box would only be an image.
Basically, Default is when the numbers seem to outline a wall, box is where the numbers use the outline of an image wall, and top down is when Reserve and In-Clip are in completely seperate boxes. Garm3n 7MF/4MP is a "Top-Down" even though the boxes are left-right.
200 = Minigun
6/24 = Revolver
12/36 = Scout Pistol
40/100 = Syringe Gun with some expended
8/24 = Stickybomb Launcher.
4/20 = Rocket Launcher (it's a TOP DOWN, remember)
120/200 = Syringe Gun, max clip size and some Ammo Capacity
Inconsistently: It's inconsistent because even though the centering looks nice,
the ammo IN clip was centered to the left. Consistent type makes it look consistent, but it could also look subjectively ugly.
Another thing I'd like people to watch out for is number consistency. I'm saying to make the style identical but the colors the same. For example, there are a LOT of numbers and bars in a HUD.
So here are numbers:
(This kind of turned into a rant. I put a spoiler tag on it)
If I'm a Demoman with heads, a killstreak weapon, health, grenade launcher, and stickies,
it's annoying to keep track of which one it is if you're using an inconsistent HUD. It feels kind of minor, but still.