To those bashing Entei, he is one of the best CM fire types in the game. He is now outclassed by Victini, however. Entei also has MORE physical bulk than Arcanine or Victini, meaning with investment he can tank a lot of hits that neither of those can, and unlike Arcanine can stay in after they switch. I'm also fairly certain that Arcanine gets neither Lava Plume nor Eruption, although if you're going all out Eruption Typhlosion is superior to Entei.
Entei is similar to Mew. It has a lot of sets it can run, most of them are outclassed by something or another, but you can throw people off by running obscure sets they don't see coming. It's actually really easy to lure in their physical wall expecting to be able to set up on Entei. Also note that although Arcanine does outspeed with Jolly, you still are hitting just that little bit harder than Arcanine, and as was proven last gen (Electivire) it can be just that little bit that's necessary for a OHKO.