If I was to bet serious money, I would have to put it on Honda, and if I were to put it on a specific rider, Rossi. That certainly is the safest bet. However, I think that the chance for one of the others to upset that this year is much stronger than has been the case recently.
Honda clearly do have the deepest roster of top-level riding talent, with at least 4 riders who are capable of winning GPs, or even championships (Rossi, Biaggi, Ukawa and Katoh).
I don't think that the 2003 version of the Honda GP bike will be significantly different -- from what I hear, it will be minor evolutionaly tweaks, with some additional effort focused on weak areas (ie slipper clutch, etc). If there were to be a major redesign (eg the rumored V6, or more recently rumored V3), much of what they have learned in testing ofer the past several months would not be of much benefit. If they DO get beaten,or even seriously challenged this year, all bets are off for 2004, though.
My ranking of probabilities for 2003 success by manufacturer are:
1) Honda
2) Yamaha
3) Ducati
4) Aprilia
5) Suzuki
6) Kawasaki
7) Proton
8) WCM (R1-based prototype)
Proton is that far down only because their new engine has only first run under its own power about a week ago. The team's past success and experience would otherwise move them ahead of Kawasaki and maybe even Suzuki.
My gut tells me that Barros and the M1 just might take it this year, or at least mount a serious championship fight, and my heart says Ducati or Aprilia.