Arcanine finds itself in the unfortunate limbo that is BL. It isn't a bad Pokemon by any means, but it has significant issues with Pokemon that are on nearly every standard team. Its great stats and Intimidate help it a little if you're willing to use it in standard play, but Arcanine's main problem is its small movepool and lack of useful STAB to complement its great Attack stat. One of the main reasons to use Arcanine is its access to ExtremeSpeed, which helps against Alakazam, Ninjask, and users of Flail and Reversal.
Howl first, maybe twice if Arcanine survives a hit, then take foes down. Steel-types get a taste of its Fire move: Flamethrower provides a balance of power and accuracy, while Fire Blast is a viable option if you don't mind trading accuracy for some extra power. Overheat is useful if you intend to play Arcanine as a hit-and-run Pokemon; though it has a higher damage output the first time it is used, it becomes successively weaker.
In terms of EVs, focus on Attack and HP; Speed isn't really important as Arcanine will be using ExtremeSpeed a lot.
Iron Tail will OHKO all Gengar that run Defense-hindering natures, and even some that don't. ExtremeSpeed allows Arcanine to attack first, reduces its need for Speed EVs, and even lets it counter Reversal and Flail users. Iron Tail damages Rock-types such as Golem, and surprises the common Solrock switch-in with an OHKO. Overheat lets Arcanine dispatch Skarmory, who would otherwise wall this set, while Double-Edge is Arcanine's most powerful physical move. As Arcanine's movepool is so limited, it can use Hidden Power Grass over Iron Tail to OHKO Golem, and run Double-Edge in the third slot, However, Arcanine would then lose a lot of power against Slowbro and Slowking.
Again, maximize Attack, and run some Speed as well. 244 is a good Speed stat to hit as it beats all Tyranitar; 263 beats all Magneton, Skarmory, and Solrock; and 265 most Gyarados. Put the rest in HP for optimum bulk.
Fire Blast away, use Hidden Power Grass on Golem and Quagsire, and Crunch on Psychic-types, particularly Thick Fat Grumpig. ExtremeSpeed finishes off weakened Pokemon and users of Reversal or Flail. Hidden Power Ice hits Dragon-types and Gligar harder, but it's not especially useful as most of them are faster and generally pack Earthquake. This Arcanine is perfect as a late-game sweeper, as it has the potential to finish off unsuspecting, weakened teams. In standard play, though, you'll find that it gets walled by bulky Water-types and special walls, namely Snorlax and Blissey, way too easily, making Toxic or Hidden Power Fighting options worth considering. Even then, however, it won't be too effective.
Give Arcanine as much Special Attack as possible and a few Speed EVs to outrun Tyranitar; the rest optimize Arcanine's bulk.
Body Slam's paralysis chance is always a nice attribute, especially if Arcanine is paired with other slower sweepers. Toxic is a generally great move as it greatly handicaps bulky Water-types and helps against Salamence, Gyarados, Solrock, and quite a few other threats. Thief can remove the Leftovers from bulky Pokemon such as Slowbro and Blastoise, and Safeguard can be used to prevent status conditions.
Checks and Counters
In standard play, all Water-types work, as they fear only the rare Toxic. Slowbro, Suicune, and Milotic shrug off Arcanine's attacks and can retaliate with super effective STAB Surfs. Swampert has Earthquake as well, but should beware of Hidden Power Grass. Salamence and Gyarados also resist Fire and pack Earthquake, which will 2HKO Arcanine or OHKO it if backed by a Dragon Dance or Choice Band boost. As Intimidate lowers Arcanine's Attack, even Choice Band ExtremeSpeed will do pitiable damage. Tyranitar should avoid Hidden Power Fighting variants, but its STAB Rock Slide will shut down Arcanine. Blissey and Snorlax wall the special attacking variants.
If Arcanine is allowed in UU play, Water-types still pose an issue. Blastoise, Walrein and other Water-types that pack defensive spreads work well for the same reasons Milotic and friends do in standard. Poliwrath doesn't always pack Water moves or Earthquake, but if it does, it can be counted on as well. Slowking is weak to Crunch, but has great Special Defense and can Surf or Thunder Wave Arcanine before it falls; the same goes for Hypno with Thunder Wave. Solrock and Lunatone resist both ExtremeSpeed and Arcanine's Fire STAB of choice, but they are weak to Hidden Power Grass and Crunch, so beware of that.