Which symmetric algorithm should I use for storing files in untrusted servers? I have gigabites of files, which I have to backup. I have some servers for storing this files. But these servers are untrusted. So I need to encrypt files, then backup files. When my PC  breaks down, I will dowload files from unstrucsted servers and restore them. 

So, basically, I think about AES-256. Is it OK for me, or there is more suitable symmetric algorithms?
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.