Depending on what you meant by this question, I’d say Perplexity. It’s got access to a number of different LLMs, and cited its sources. The biggest concern I’ve had when it comes to LLMs is that they eventually make shit up. If you can verify its answers by checking its sources, you have a much higher confidence level in the answer.
I’d argue that Perplexity is in this camp as well. It’s my go-to for anything more open-ended or for multi-step research I need to do. No need to wade through garbage search results, and it cites its sources. Kagi seems to be doing very much the same thing, but in addition to their paid search engine service.
I don’t know that I’d switch from Perplexity, but I could see an argument for it.