Hm..they're all quite competent and talented, but since Gary is a researcher, he may be a bit 'behind' in terms of experience by now. You never know though.
As for Richie...he defeated Ash in the Indigo League and is generally pretty smart I guess, but if he has about the same team as when we last saw him, I don't think he'd stand a big chance against the majority of the other trainers listed there.
Ash is an excellent battler, I don't even need to restrain my fangirlism to know that it's fact ;P He handles all of his Pokémon very well, and under normal circumstances (without these sudden over-confident/snobbish moments the writers seem to love so much), he can beat just about anyone who isn't an Elite Four member, especially with the awesome team he has right now.
I wish we could see Drew having full battles with no time limit for once, since those are the type of battles who make one's strength most obvious IMO. He's a great battler, no one can deny that, but I'm not sure if he has the stamina to keep the same pace and force in a full 6VS6 battle that could possibly be more than one hour long. He seems to, but I can't tell.
James has defeated Ash before (I didn't see that particular episode though), but I think that was with rental Pokémon or something. He just needs to be far away from Jessie and stronger Pokémon in his own team, and I'm sure he'd be able to give anyone of the above mentioned trainers a run for their money.
Torn between Drew and Ash right now, so I'm not voting. I might do later though.