Llama 3.1 70B Instruct is considered the best overall open source LLM, offering a large context window and excellent retrieval benchmarks, along with strong code generation and mathematical reasoning capabilities. It has some limitations, such as only supporting eight languages and having a custom commercial license that applies to fine-tuned derivatives. On the other hand, Llama 3.1 405B is a powerful big LLM that rivals heavyweights like GPT-4o, with a massive 128k-token context window for retrieval-augmented generation and tool use. However, it requires significant computational resources and has a custom commercial license. The best small LLM under 7 billion parameters is Phi 3 Mini, which offers excellent output quality and permissive MIT licensing. Mixtral 8x7B is another great option, with high output quality and efficient inference on A100 GPUs. Zephyr models are the best aligned chat LLMs, offering helpful assistant behavior and supporting ChatCompletions-style roles out of the box. Code Llama is the best model for code generation, with strong performance on evaluation benchmarks and four sizes to choose from. Llama 3.1 is a great option for fine-tuning projects, with flexible base models and explicit licensing for derivatives. Ultimately, the best open source LLM depends on specific use cases and requirements, and it's recommended to experiment and find the right model for each task.