Question

I`m searching for an Encryption Method better than MD5. It must be one way encryption because I do not want to use keys in encryption.

Answer 1

Basically MD5 is a Hashing algorithm not an encryption.
you can use more stable SHA1

using(System.Security.Cryptography.SHA1 hash = System.Security.Cryptography.SHA1.Create())
{ 
   System.Text.ASCIIEncoding encoder = new System.Text.ASCIIEncoding();   
   byte[] combined = encoder.GetBytes(str);   
   hash.ComputeHash(combined);   
}

EDIT: answer to comments

System.Security.Cryptography.SHA1 Class

The hash is used as a unique value of fixed size representing a large
amount of data. Hashes of two sets of data should match if the
corresponding data also matches. Small changes to the data result in
large, unpredictable changes in the hash.

The hash size for the SHA1 algorithm is 160 bits.