You get a tank in nagrand (if you build the tankworks) for the entirety of the zone (98-100 leveling)
Terrible advice. Get the stable. You can summon a talbuk and engage in combat while riding it. It's 10,000 times better than the shitty tank.
Large Building: Barracks then Bunker
Medium: Lumber Mill, then Inn
Small: Profession, then Salvage Yard, then Profession
Reasoning
You want shit that improves your Garrison, then stuff that improves your Followers ASAP. High ilevel followers let you go on high ilevel missions which give you Timeless Isle style purples occasionally, and a chance at free Highmaul gear once a week.
Barracks
The barracks provides Benefit at Level 1 and Level 3, with a nice utility at level 2 that can give you some specific benefits, like Repair and Follower Mission assignment when in Draenor.
Level 1 bunker opens up a new mission type that awards higher follower XP, and more missions being availible.
Level 3 bunker allows you to have 5 more followers active. Normally you can only have 20 followers active, so a level 3 bunker gives you 25% more followers to do shit with.
Bunker
Level 2 bunker opens up work orders for garrison resources. The bunker eats resources at the same rate the Lumber Mill produces them. The bunker work orders seem to have around a 25-33% chance to reward follower upgrade items, and all work orders in general seem to have a chance at Apexis Crystals and Abrogator Stones (legendary ring quest items)
Level 3 bunker gives you one of your Seals of Fate (warforged seal for WoD) for free, a nice bonus.
Level 1 bunker is not very useful as it gives a chance at upgrading quest rewards, which won't be very strong as we have SoO gear.
Lumber Mill
Level 1 lumber mill opens up work orders for timber gained from chopping down trees
There doesn't seem to be much of a benefit to leveling lumber from 1 to 2, as 7 work orders takes 28 hours IRL to accomplish, harvesting larger trees lets you get more lumber quicker, but the choke point is the 4 hour work order, not harvesting.
Inn
The Inn will be one of your most important long term buildings.
At level 2, Once a week you can recruit a follower with 1 trait of your choosing.
Level 1 unlocks daily dungeon missions which can reward toys, and level 3 unlocks Treasure follower missions which have increased gold rewards.
Don't get the Storehouse Small building, it sounds like a good idea on paper (more work orders) but work orders take 4 hours IRL to complete, so just having a larger queue is worthless.
Salvage Yard
The salvage yard gives a chance at your followers giving you a crate of crap at the end of a mission. At level 2 and above these crates can start containing Follower Upgrade items, and at level 3 can even contain world drop BOEs