Haha, I like all of them, but just to be slightly different...
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Byron's pokémon are good in both games, although I think despite the commonness I prefer battling Bronzor to Magneton (Although he'd be even better if he had Bronzong). Bastiodon is a cool pokémon and kinda cute, and Steelix is tough too. He makes a pretty tough battle, but normally quite straightforward, although failing to OHKO Bastiodon results in pain. He's also just a nice character overall.
Fantina also has cool pokémon, such as Mismagius, Gengar, and in Platinum Drifblim, Basnette and my personal favourite Dusknoir. As is the nature of ghosts, they're tricky to face. Also, she's not creepy like most ghost users (the main reason I like Phoebe from Hoenn), even if she is slightly French (I dislike the French accent and most of their food, although not croissants).
I can't understand why these guys don't get more love...