Answer: The most secure Diffie-Hellman group is currently considered to be Group 24 (2048-bit ECP) or higher, offering stronger encryption and resistance to attacks.

The security of a Diffie-Hellman (DH) group depends on the size and type of the underlying prime numbers or elliptic curves used.

Group 24 (2048-bit ECP)

This group uses elliptic curve cryptography (ECC), which provides high security with shorter key lengths, making it efficient and secure.

Group 14 (2048-bit MODP)

With a 2048-bit modulus, this group offers a solid balance between computational requirements and security, resisting most known types of cryptographic attacks.

Groups 15 and 16 (3072 and 4096-bit MODP)

These groups offer higher security levels due to their larger key sizes, making them more resistant to attacks but at the cost of increased computational overhead.

Group 18 (8192-bit MODP)

This group provides extremely high security levels, suitable for environments where protection against future quantum computer attacks is considered.

Conclusion

The most secure Diffie-Hellman groups are those that use large prime numbers or elliptic curves, with Group 24 (2048-bit ECP) being among the most secure due to its efficient use of ECC.