Entei is not a Pokemon I would recommend for VGC 2016. The main reason to use it, Sacred Fire, is completely nullified by Primordial Sea (while Will-O-Wisp, a move many of Entei's competitors feature, can still be used), and the premier threat in the format, P-Groudon, can't be burned. Entei does still fill a solid role in Battle Spot Doubles, just like it did in VGC 2015.
Entei is really hurt in VGC by the Pentagon-only rule, as it loses out on Extreme Speed, but it already faces stiff competition from other fire types, especially Arcanine, who has a much better ability, better recovery, and similar stats (while again, still being able to burn things in heavy rain if you play under this year's rules). Entei's niche as the only non-restricted Pokemon to have Sacred Fire, however, still makes it a viable option.
Entei @ Leftovers
Ability: Pressure
Level: 50
EVs: 236 HP / 252 Atk / 20 Spe
Adamant Nature
- Sacred Fire
- Stone Edge
- Protect
- Iron Head/Bulldoze/Snarl/Substitute
Max attack with an Adamant nature maximizes Sacred Fire's power. 20 speed EVs were chosen to outspeed Bisharp and KO it (or KO and hopefully burn it if it has a Focus Sash), but more speed can be invested to outspeed things like the less common Breloom. Entei does have base 100 speed, so a Jolly nature with max speed can be chosen if you want, but you'll be flipping a coin to outspeed threats like max speed M-Kangaskhan (and then flipping another coin that you even get the burn). Stone Edge provides coverage, especially against Charizard and Talonflame, and Protect is Protect. Iron Head and Bulldoze can give additional coverage against fairies or things like Heatran, and Bulldoze can also help provide speed control against things on the ground. Snarl and Substitute work well with Entei's bulk, and Leftovers further increases this bulk, but items like Assault Vest, Sitrus Berry, Safety Goggles, etc. are nice alternatives depending on what your team needs.