First before I begin this post, I will give credit to the original user who posted a maximum theoretical loot calculation last year, This is merely a revision of those calculations as since that post, there has been the addition of Level 12 Collectors in the December 2014 update, the introduction of clan perks during the February 2015 update and also the Titan/Legend Leagues in the July 2015 update.

Firstly, lets begin with the Gold/Elixir stored in the Town Hall:

1,000 Gold
1,000 Elixir

Storages:

Storages have a cap of 400,000 gold or elixir when being attacked, whilst Dark elixir storages have a cap of 3,000 Dark Elixir.

400,000 Gold
400,000 Elixir
3,000 Dark Elixir

Resource Collectors:

A Town Hall 10 has a maximum of 7 of each resource collectors excluding Dark Elixir drills.

Each resource collector has a maximum capacity of 200,000.

Up to 50% of each resource can be stolen from each collector.

0.5 * 200,000 * 7 = 700,000 Gold
0.5 * 200,000 * 7 = 700,000 Elixir

Dark Elixir Drills have a maximum capacity of 1,800.

Up to 75% of the Dark Elixir within a drill can be stolen.

As mentioned earlier, there are 3 Dark Elixir Drills.

0.75 * 1,800 * 3 = 4,050 Dark Elixir

Clan Castle:

5% Of a TH10’s CC loot can be stolen.

A Max clan castle holds 2,000,000 of each gold/elixir and 10,000 dark elixir however, if the player is in a level 10 clan, the war loot extra storage bonus of 50% means the clan castle can actually hold up to 3,000,000 gold/elixir and 15,000 Dark elixir.

0.05 * 3,000,000 = 150,000
0.05 * 15,000 = 750

Total:
1,251,000 Gold
1,251,000 Elixir
7,800 Dark Elixir

Total including league loot bonuses:

If you take into account the Legend League loot bonus of 250,000 Gold/Elixir and 2,000 Dark elixir, Theoretically, the maximum amount of loot that can be gained from a single attack is actually:

1,501,000 Gold
1,501,000 Elixir
9,800 Dark Elixir

If I have made any mistakes, please correct me. Hopefully this is helpful if you ever wondered what the maximum amount of loot is.

Note: The maximum theoretical loot may increase as new updates are implemented into the game. Perhaps, when this happens, a new updated guide should be made and tagged with the Date eg. [Sept 2015] and eventually we'll be able to see how the maximum theoretical loot increases over time by comparing these posts? Just an idea.