Clash Of Clans Essentials : Spend Your Gems Wisely

What to spend your gems on heavily depends on how much you play the game, and how much you're planning to spend/earn. If you play the game pretty often, like raiding multiple times per day in steady blocks, prioritize this way:

1. Builders Huts - the number and availability of builders is the biggest brake on your progression. Everything else pales in comparison.
2. Early Completion of Upgrades - you want to speed up upgrades that finish when you won't be able to immediately start the next one, and to free up a builder if you max out your storage capacity.
3. Boosting Collectors/Mines - If you're online a lot, this is the most efficient way to earn more of a resource outside of/in addition to raiding. The cost is relatively low, and the benefit pretty high, especially for those harder-to-raid resources like Dark Elixir. If you did 1 and 2 though, you probably won't need it because you'll always be broke anyway.

If you're not online a lot, or play the game casually, builder availability is most likely not your biggest issue. Instead, it's accruing resources, and keeping them. As such, spend this way:

1. Boosting Barracks - Let's face it, you get attacked all the time, and you're hemorrhaging trophies and resources every time. So maybe you get an hour every couple days or week where you can just play solid. Boosting the barracks will allow you to turn around armies in the time it takes to make an attack, and you'll rapidly earn both trophies, and the resources to get your builders occupied. Also, it's just fun.
2. Buying Resources - Buying gold/elixir/dark elixir straight out will allow you to "close the gap" between what you can raid and what you need for an upgrade. Casual players get raided all the time once they get close to full storages, and so can never keep them that way. This way you can play in bursts, and get as many high-cost upgrades going as possible. This is especially critical if you've rushed your TH (detailed later).
3. Builder's Huts - They're still pretty awesome, and there are enough low-cost upgrades like mines/collectors that you can have excess builders working on, at least early in the game. Later on though, you may find these guys sleeping more often than working.