Okay, so I have tried using the legendary beasts on teams (seperately, I'm not that much of a masochist), I'll give my thoughts on them, as well as the sets I used.
Raikou @ Life Orb
Ability: Inner Focus
Tera Type: Fighting
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Aura Sphere
- Volt Switch
- Scald
- Shadow Ball
This has been the best legendary beast that I have tried so far, it's coverage and stats lend it quite well in the OU metagame. Being an electric type that can hit ground types for super effective damage is amazing, as you can easily predict the switch in, hit a scald and then another to ko them, meaning you can volt switch for free. Aura sphere allows it to deal with archaludon, kyurem and gambit quite well. It can be a powerful offensive threat, and it shined immensly.
Entei @ Heavy-Duty Boots
Ability: Inner Focus
Tera Type: Normal
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Sacred Fire
- Extreme Speed
- Scorching Sands
- Crunch
And now we go on to the worst legendary beast. While its speed tier is nice, it's power output is not great without band, which severly restricts its flexibility. Tera normal extreme speed's don't even ohko barra, which do a max of 80%, which is just pathetic. Sacred fire absolutely carries the set, while scorching sands allows you to deal with arch and dondozo better. The last moveslot is mostly free, I chose crunch because dark is a good offensive type all around. It will definetely get better if rain gets worse, but that 115 is not good. If it had something like morning sun, which gouging gets but it doesn't, then it could be better but alas.
Suicune @ Leftovers
Ability: Pressure
Tera Type: Steel
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Ice Beam
- Scald
- Protect
- Mirror Coat
Suicune is alright when used in OU, it is a bulky water type that can deal great damage. I've found that mirror coat is a great move on this mon to destroy some special mons as tera'd it can take a lot and hit back with a ko. The combination of ice + water is great and scald burns can mean some water mons don't want to switch in. Not a bad choice, but it is outclassed by other mons such as primarina.