Garrisons are awesome, but what buildings are best for your playstyle can be a little confusing at first. So I have decided to ease the minds of those who don’t know what to do with them by listing what my thoughts are on each of the buildings and why I chose the ones I chose.

What I would choose: Frostwall Tavern, War Mill, Goblin Workshop, Trading Post, Scribe’s Quarters, Enchanter’s Study and Salvage Yard.

Small Plots
Profession Buildings (Enchanter’s Study, Scribe’s Quarters, Alchemy Lab, Engineering Works, Gem Boutique, Tailoring Emporium, The Forge and The Tannery):
I grouped these together because this fits for all of them – get your profession’s applicable building(s). The work orders you can put in will help make crafted gear and the connected upgrades. I find it makes very little sense to upgrade past level 2 unless you don’t play for several days at a time, and since I log in daily they’re staying at 2. Plus, 2 is when you can assign a follower to work there, which allows for some cool additional functionality sometimes.
Storehouse
Bullshit, waste of a plot. A level 3 garrison has a portal to Warspear which has anything you can get out of a storehouse. Add in that level 3 requires that you loot 10,000g in draenor, I have no idea why anyone would want this.
Salvage Yard
This I like. Allows for salvage to be gained when you succeed in follower missions. This salvage can contain anything from random trash to Highmaul raid gear.

Medium Plots
Trading Post
This can be a major tool for gathering resources. It takes random trade goods (a specific herb, ore, etc.) and converts it directly into resources through work orders or vice versa through a vendor, and you can get that at level 1! Level 2 gives you access to a couple of factions that are more flavor than functionality as well as the ability to gather parts for a mechanical auctioneer, and level 3 increases reputation gains in Draenor by 20%.
Barn
I haven’t built one, but level 1 appears to be strictly material gathering for leatherworking and tailoring and level 2 for cooking. Level 3, however, lets you harvest Savage Blood, which is super expensive and is used in all profession crafting.
Frostwall Tavern
This is good for people who need daily dungeon quests and people who focus on follower missions. Level 1 has random dungeon quests (a couple with pretty nice rewards, including a reusable item that increases all stats by 200 for an hour with a 15 min cooldown and another that gives you an epic 100 follower). Level 2 allows you to recruit a follower (level 90) weekly that you can pick by what traits they have (need a follower that can defend against, say, group damage? Recruit one here). Level 3 unlocks Treasure Hunter follower missions (I haven’t upgraded this far yet).
Gladiator’s Sanctum
People get this for PvP quests. I don’t care about PvP. There are a couple of interesting things that it grants you like safe fall and underwater breathing in Draenor at level 2 and damage reduction in outdoor areas at level 3, though. Kinda ‘meh’ for me with how I play.
Lumber Mill
I kinda wanna shit on this one, but some may like it. It’s strictly for resource gathering, nothing else. You feed the work orders with lumber you gather from around draenor and it’s really boring. The only reason I would get this again is for a powerful item that allows you to break combat, but I’m pretty sure you can get this at level 1 and then replace the mill with something better.

Large Plots
Barracks
This is the first building you are forced to build. Level 1 allows for Patrol follower missions (I bet you didn’t even notice doing them). Level 2 allows certain followers to follow you around Draenor, which could be cool if you want to feel like a BM hunter. Level 3 increases your active follower limit from 20 to 25 (I haven’t upgraded to this). It feels like it’s a little too lackluster to keep, at least before you have a level 3 garrison.
War Mill
I like follower missions, and this is BIG for followers. Level 2 lets you feed resources into work orders to make follower gear, and the only way you’ll get anything useful from follower missions is if you upgrade their gear. It’s also good for transmoggers who like to look pretty (Panzer).
Goblin Workshop
This is actually a pretty fun one. You get temporary items that do fun things like launching you up in the air (outside of PvP battlegrounds), but also practical things like damaging enemies and guarding you. Once you get to level 3 you can use a siege engine in Draenor outdoor zones.
Spirit Lodge
To be honest I haven’t tried this one, but from what I have read it’s just a transportation system that adds portals from your garrison to various places around Draenor. Worth a large plot?
Stables
I haven’t tried this one, but it seems to allow you to stay mounted when otherwise you would get dismounted, not get dazed when you get hit by mobs while mounted and increases your mount speed by 20%.

That should be all of them. If you have any questions on any of them I’ll help where I can.