Hands down, Marley's episode. Not only did Brock get some much-deserved spotlight and humor, but Marley's snarkiness was incredibly fun. Her "I'm an alien" line was the first joke in DP to actually make me laugh out loud.
I actually wasn't particularly fond of any of the episodes. For me, the Stat Trainers' presence was the only memorable aspect about those episodes.
Mai. Her episode was probably the most awkward Pokémon episode ever. I mean she first appeared asleep in a wooden boat randomly floating down a river and Satoshi and Hikari for the first time ever have second thoughts on helping a COTD. I guess those reasons make it unique. :-\
''A single thread in a tapestry though its color brightly shine, can never see its purpose, in the pattern of the grand design.''
Marley was a fun character, but Riley's episodes were my favorite, mainly because of the Galactic stuff, and Lucario.
Look Forward, Team Rocket!
Also I claim Piplup in the Claim a Pokemon thread =)
Riley, with Cheryl running close second, but that isn't really his credit, rather, the episodes he was in were what was good.
Cheryl was interesting personality wise, but only one episode of her mini-arc was interesting.
Mira's had nothing to do with her in-game part and was really wtf-ish.
Marley was a bit bland, and not in the way it was intended.
Buck was forgettable as hell, and didn't do anything really.