Decrypting DES with decrypted and encrypted data I got two 8-byte strings.

One which is decrypted is:

FF FF FF FF FF FF FF FF


and one which is encrypted is:

16 05 78 B0 0A C2 78 7F 


Encryption mode is CipherMode.ECB

I need the key used in DES encryption.
Do you know anything at all about the key?  If not, then what you have is a "Known plaintext attack" - you know the cyphertext is 16 05 78 B0 0A C2 78 7F and the plaintext is FF FF FF FF FF FF FF FF.

DES has some weaknesses, but it isn't that weak.  A cypher that can be cracked with a mere 8 bytes of plaintext/cyphertext would be very weak indeed.

Can you generate more cyphertext/plaintext pairs if you want to?  That might allow you to use linear attacks and differential attacks.

Apart from that, you will have to try to brute force all possible keys and see which one works.