SCARY DRAGON
Flash fire only raises fire moves, and most of the time, the fire move will be from another poke, so the boost is almost useless most of the time, unless you manage to kill the other guy AND they don't have something that resists fire or that can kill him before they kill arcanine. Thunder fang has extremely little power and is really only useful for gyarados. Infernape has a powerful STAB move that somewhat supports his fire part at least.
What I'm saying is that it still allows you to switch into any fire move... also, thunder fang can deal decent damage to most water pokemon, although, Arcanine ((and any other fire type)) is probably already dead to those anyways.
Also, Infernape is a piece of crap... the only good item for him is a choice item, or a focus sash. They sacrificed too much durability when trying to make him have both physical and special attack.... unless the infernape is packing Earthquake, Arcanine can 2HKO it with Espeed any day.