In terms of individual components of Ash's group, Brock can never be replaced. And I know people are gonna jump all over me for saying this, but in my opinion, Dawn was a much better female character than Misty was. Say what you want about Contests, but at least it gave May and Dawn a purpose for being on the show. Misty really didn't contribute all that much. Dawn was the most developed out of the female protagonists so far and she went through such an amazing journey in just one region. I felt sad to see her go more than I ever missed May.
Now, as for a group dynamic (because I know that was a little off-topic) the original trio - Ash, Misty, and Brock - were and always will be the best. Together, when you're not considering them individually, they had a dynamic that worked really well together and the URST between Ash and Misty was always great to watch. So that's my two cents, take it or leave it.