SPOILER ALERT! If you haven't heard about which characters are present in the DP Anime, and which aren't, stop reading now
I voted Brock, but there's a good chance that could switch to Misty over the next few days. It was a tough choice. Back in the day, I sort've got sick of Misty being around and welcomed the change to May with open arms, but in retrospect I now realize how much I missed having her in the group. May was just... she started off interesting because she was fresh and new, but by the end I can't say I was all that upset to see her leave (unlike "Gotta Catch Ya Later!", which made me cry when I first watched it). To be fair, though, that may have been less about May being an inadequate character, and more about Max leaving with her
May had her moments ^^
Now, why did I vote Brock? I don't deny that he's been pretty damn boring over the Hoenn saga, only really having a moment to shine every 30 episodes or so, but he has a powerful presence on the team. I loved Tracey, but Tracey was no Brock (he just tried to be). Brock is the sound of reason... as long as a pretty girl isn't involved. He's also Ash's best friend aside from Misty, and losing both of them would really suck. Now that Max is gone, Brock returns to being the sole "smart one" of the group, too. Then there's the fact that Brock is the oldest member of the crew, though I suppose Tracey was an "older" member too... Can't have two males around the same age on the team, eh?
It was close, but for all those reasons and more, I think that Brock is a vital member of the crew. There's always room for development if the writers put the time in, and hopefully with the group back to being a trio in DP Brock'll get more attention.
So Wiii. Go Brock! But if Misty comes out on top, I won't disagree ^^
/SPOILER
EDIT: I have a few weekend poll ideas of my own, actually.
Ash vs Gary OR Red vs Blue (the characters, not the games) - and just because.
Best Christmas Pokemon: Stantler vs Delibird