I just thought I'd throw in my two cents about Entei.
For competitive use, which I will define as Smogon UU, Entei is mostly outclassed by the other three prominent Fire-types in the tier: Arcanine, Rotom-H and Infernape.
Infernape has access to a speedy Stealth Rock, priority in Mach Punch, and the ability to run a fully Physical, fully Special, or Mixed set. Arcanine is slightly less bulky and slightly less speedy, but he has a much better movepool and access to Intimidate, as well as recovery via Morning Sun. Rotom-H has Levitate to eliminate Earthquake weakness, as well as superior bulk and slightly better defensive typing than Entei. For these reasons, Entei is kind of overlooked in competitive battling. He has some nice things going for him in Sacred Fire, Extreme Speed and fantastic HP and Attack to back them up, he has no reliable recovery outside of Rest and a sad movepool of mostly Special Fire attacks.
But wait! Has Entei found a home? Yes he has, your loviest Fire Dog has found a niche in the VGC format! His buddies Raikou and Suicune have already nestled in quite well, and now they can be a trio once more. I know you have some questions; let me answer them
What's so good about Entei now?
Entei has several advantages now in VCG. For one, his base 100 Speed stat can be buffed by Tailwind, which is much easier to set up in VGC because the setter's partner can take advantage of it on turn 1 and you don't have to waste a turn switching. He also gets his awful movepool trimmed down one attack with Protect, meaning that you only need three cruddy attacks instead of one!
Wait, what happened to his competition?
Infernape is kinda irrelivant in VGC, so that really just leaves Rotom-H and Arcanine. While Entei doesn't, and will probably never, rise above them, he has elevated to a level en par with these two.
What does Entei check in VGC?
With VGC comes Mega Mawile, and with Mega Mawile comes the need for Fire-types. She can't scratch the stuff. While there are plenty of answers to MAwile already, Entei can occasionally rise to fill a spot that none of the other guys can. He has a faster Will-o-Wisp than Arcanine, as well as a non-recoil Fire STAB witha 50% chance to burn. This lets him shut down Physical attackers like Mawile and Mega Kanga much more efficiently, as well as being able to handle Mega Venusaur one-on-one if it lacks Leech Seed. Also, what with Bisharp and Competitive Milotic menacing VGC, having too many Intimidate users can be a hinderance to your team rather than an improvement. Therefore, if you already have Landorus-T and Gyarados, but you need a Fire-type, Entei fits the bill much better than Arcanine can. Entei also has the added benefit of beating most Fairy types (bar Azumarill) in head-to-head combat between Sacred Fire and Iron Head. He can also offer risky Speed control with Bulldoze.
Wow, thanks, Entei looks like a total rekking ball!
Not so fast. Unfortunately, Entei has not had access to Extreme Speed since Gen IV, meaning that he lose one of his main selling points. Arcanine, on the other hand, does have access to E-Speed. Entei also suffers from accessability issues. Anything you use in VGC has to be caught in-game, meaning that getting your perfect Nature, 6IV Entei will be a huge undertaking. This is another reason why Arcanine is used more. Entei is also weak to common spread moves like Rock Slide and Earthquke, as well as Intimidate. He is hard countered by Landorus-T and Rotom-W, two very prominent threats.
So how good is Entei, exactly?
Entei is the best Pokémon evrEntei is decent, but more often than not outclassed by his Fire bretheren, namely Arcanine. While he does have roles to fill, he is a very niche Pokémon.