Entei

Type Tier
Fire BL
Statistics
Stat
HP
115
433
Atk
115
328
Def
85
268
SpA
90
278
SpD
75
248
Spe
100
298
Name Item

Sunnybeamer

Leftovers
Moveset
~ Fire Blast
~ Solarbeam
~ Sunny Day
~ Return / Rest

This set turns weaknesses into strengths. Waters will have greater trouble dealing with this due to Sunny Day, and Solarbeam punishes Water, Ground, and Rock-types, all of which are common Fire counters. Fire Blast over Flamethrower lets you deal the optimal amount of damage in the shortest period of time. Return if you want to have a solid finisher move without depleting Fire Blast PP or if Sunny Day isn't active, Rest if you want Entei around for the long haul.

Other Options

Hidden Power Fighting can punish a lot of Pokémon that would normally switch into Entei, including Blissey and Tyranitar. It is sturdy enough to be used as a pseudo-hazer against Perish Trappers and the like. If you favor accuracy over power on a set, Flamethrower works out fine as well. A Curse or Sleep Talk set are viable if you need one or the other on your team.

Opinion

Being a Fire-type and having a horrible movepool hurts Entei badly. It doesn't learn any good moves that works off its STAB or best stat either. But it is redeemed a bit by being able to survive a lot with its massive HP. Entei is the only bulky Fire-type in this generation, which makes its usefulness unique, albeit limited. It is legendary only by name, as it is by far the worst of the legendaries available. Because of this, it is only viable in the UU metagame (if it is allowed there), as anything in the OU tier would tear it apart quickly and without hesitation.

Counters

Tyranitar, although weak to Solarbeam, takes the hit well and can dispose of Entei without too much trouble. Most Rock and Water Pokémon, in particular Starmie, also counter Entei if Sunny Day isn't set up due to their ability to naturally counter Entei's type.