Hm. Considering that people are posting their garrison setups here, I might as well throw in my two cents...
Keep in mind, this is for my main, who is a blacksmith/miner, protection paladin with the intention of raiding and keeping my blacksmithing as supplied with ressources as possible.
Garrison lvl1:
Large plot: Barracks
Small plot: Forge.
Garrison lvl2:
Large plot: Barracks
Medium plot: Most likely the Lumber Mill, to have as many resources as possible. The other options considered are Trading Post and Lunarfall Inn, them being the final targets for the medium plots; however, Lumber Mill is preffered until enough resources are gathered.
Small plots: Forge, Salvage Yard or Storehouse - I am yet unsure which one of the two I should put first in the aditional slot.
Garrison lvl3
Large plots: Barracks, Stables (I just can't resist the thought of seeing my mounts stabled there x) )
Medium plots: At the start, Lumber Mill in one of the plots; once enough ressources are gathered, Lunarfall Inn and Trading Post, probably Trading Post being first for the aditional plot.
Small plots: Forge, Salvage Yard, Storehouse.
One thing I am unsure of is whether garrisons will be account-wide or character-based. While I strongly believe it will be the second one, if the first one is how it will be, then Barn will have to be worked in somewhere. Otherwise I will have barns in garrisons on my alts.