Which symmetric algorithm should I use for storing files in untrusted servers?
Just a note: AES is not a encryption scheme, it is technically just a block cipher; though it is a fine one.

To create a cipher you need to use it in a mode like counter mode. 

To obtain chosen ciphetext security (which is the usual aim) you should use an authenticated encryption scheme such as Galois/Counter Mode. 

As usual, don't try make it yourself, use a premade secure library.