Cheren: Easy with my Riolu
Roxie: Slightly more challenging with the Poisoning thing, however I had a Magnemite, so it wasn't too strenuous
Marlon: I thought he was going to be a bit of a nightmare with that Jellicent, but nah, Virizion knocked out his Carracosta quickly, Ampharos took down Wailord in 1 hit, and Ampharos took down Jellicent in two hits, after having Discharge disabled.
So in terms of "easiness", they were all easy, however I haven't struggled with any of the gyms 'cause I usually go in levelled up to two levels higher than their highest levelled Pokémon.
In terms of design and personality, I love Roxie and Marlon. Cheren still kind of bores me a bit though.