Which version of ChaCha is more secure? Which version of ChaCha is more secure?

XChaCha20-Poly1305-IETF

ChaCha20-IETF-Poly1305 

ChaCha20-Poly1305


I am looking for 256bit strength for large datasets.

I am having trouble disambiguating the delta between these ciphers available in libSodium.
The only difference between these is the nonce size (and, consequently, the internal counter size). The core function is exactly the same. They all offer the exact same security level if they are used as expected.

The trade-offs are described in the AEAD section of the documentation.

XChaCha20-Poly1305-IETF is the one that has the largest nonce size. This significantly reduces the risk of reuse, since it can be randomly chosen without any practical risk of collision. This can also be leveraged to build nonce misuse-resistant schemes.

Unless you specifically need compatibility with a different implementation that doesn't support the XChaCha20 version, this is the one you want to use in virtually all cases.

XChaCha20-Poly1305-IETF is very likely to become the function used by the high-level API in a future libsodium release. Which means that other variants will not be available any longer in minimal builds.

Since your use case appears to be encrypting large messages, the higher-level secretstream API is a better fit. It uses XChaCha20-Poly1305-IETF internally.