For me, it's Ash, Brock and Dawn, because Dawn brought so much more to the anime than the others did, at least in my opinion. It was made clear right from the outset that she was going to have a larger role in the anime than her predecessors, as the first episode of the DPP saga opened with her starting her journey, and at no point did her ambitions play second base to Ash's.
I loved that about Dawn: she DEMANDED attention. I would even argue that she had more development than Ash did; she might have had the same goals as May, but she had more to work for, in my opinion, and Zoey made for a better motivational rival than Drew did. Plus she was so GOOD for Ash, as well; he learned from her, and she from him, and it worked wonders for their characters. The spin-dodge thing was overused, but it was the most creative thing to come out of the anime in years before that (that made sense, anyway; the less said about Ash's crazy strategies, the better) and Dawn put on a spectacular show in some of her contests...and she lost with far greater dignity than May. She didn't have an irritating little brother, either.
Brock, of course, is a staple; he's been the backbone of the trios for so long it's difficult to imagine them without him. Tracy was a nice breather, but he just wasn't the same, and Cilan...well, I honestly don't care for Cilan.
So, yeah. Much as I love the original trio - Ash and Misty really did harp on like an old married couple at times - I have to go with Ash, Brock and Dawn, because Dawn was exactly what the anime needed: a fresh character who could stand toe-to-toe with Ash and make it about more than just him traipsing across the newest region.