Personally I'd say Flareon. It's not exactly the best of the Eeveelutions, but as far as raw fire(literally)power goes, it can get pretty crazy, though it can also make you limited. Mine knows Flare Blitz, Sunny Day, Will-O-Wisp, and Charm...as you can probably see it's limited, but yeah with Guts it can just deliver incredible burst damage aside from on something that resists Fire, and you have two defensive moves to help out as well. Add in something like Toxic Orb(for Guts), or a Life Orb/Charcoal/Flame Plate and then it just starts getting ridiculous and that's before the fire-centric field effects and sunny day. xD
It's kinda frail though physically(not specially which is pretty decent), which is why the Charm/Will-O-Wisp support, so.
Or, you can give it Flash Fire, which would "really" help out in Charlotte's gym, since if she used any fire moves on Flareon, it'd just end up making it even more of a monster.