I successfully got a Mikrotik router to run as L2TP server, but it looks like Windows clients would fail to connect if I remove 3DES, which is not exactly secure, from the list of encryption algorithm. I have the choices of: 3DES, AES-256, AES-192, AES-128, Blowfish, Twofish, Camellia-256, Camellia-192 and Camellia-128.
L2TP doesn't provide any encryption.You can choose to use 256-bit AES encryption. Using a 256-bit AES key could potentially offer more security against future attempts to access your files.
For example, if it would take a quadrillion years to brute-force 128-bit AES, does it really matter that it might take even longer to brute-force 256-bit AES? For all realistic purposes, they’re equally secure.