Entei w/cband/leftovers/assault vest
Adamant 252atk/252spe/4def
Sacred fire
Stone edge
Espeed
Ironhead/sub
Sub Entei is only with leftys but it would be just as bad as sub ho-oh is in ubers. Choice band does heavy damage while assvest gives better bulk. Bulldoze and flame charge are the only other viable attacks entei gets tho. Nothing other than resttalkers will appreciate a burn and firetypes won't enjoy stone edge. Iron head is for the fairies/rocks.
Any counters that might trouble this set?
Rhyperior can handle Entei quite nicely. Even with Iron Head and Choice Band, it's
almost 3HKO on 252/0 neutral nature Rhyperior.
252+ Atk Choice Band Entei Iron Head vs. 252 HP / 0 Def Solid Rock Rhyperior: 157-186 (36.1 - 42.8%) -- 95.8% chance to 3HKO after Leftovers recovery
Rhyperior can OHKO with Earthquake, too.
Intimidate Qwilfish is a great counter, too. A -1 Choice Band +252 Attack Entei 3HKO with Stone Edge:
-1 252+ Atk Choice Band Entei Stone Edge vs. 252 HP / 252+ Def Intimidate Qwilfish: 142-168 (42.5 - 50.2%) -- guaranteed 3HKO after Leftovers recovery
Other than that, Flash Fire Pokes stand out as good checks, if Entei is locked into Sacred Fire/Iron Head.
And then, there's everybody's favorite bro; Slowbro
2
52+ Atk Choice Band Entei Stone Edge vs. 252 HP / 252+ Def Slowbro: 111-131 (28.1 - 33.2%) -- 91.7% chance to 4HKO after Leftovers recovery
Entei faces a lot of competition from other Physical Fire types; Victini, Darmanitan and Arcanine.
1. Victini's ability to go mixed allows it to devastate all 3 walls above. Although it has that Psychic typing, it has a solid movepool and great stats.
2. Darm's insane Attack Stat coupled with the Sheer Force boosted Flare Blitz goes great with it's access to Superpower, Earthquake and U-turn.
3. Arcanine can't beat Entei in raw power, but it's ability to absorb fire attacks and access to recovery in Morning Sun gives it longevity which the other fire types can't boast.
Will Entei make a splash in UU? Probably nothing too extreme, but that Sacred Fire gives it a very special niche.