Groudon: His Sp. Atk is horrible. Well, horrible for an UBER! Give him Fire Punch and Thunder Punch/ice Punch. Thunder for WAter coverage, Ice for Dragon coverage.
Reshiram: Great! Maybe Wide Lens as you have Blue Flare and Focus Blast which both have bad acc.
Ho-Oh: A tank needs Calm Mind instead of Toxic. Sacred Fire should go for WEather Ball if you want special, but I'd go physical with E-Quake and Brave Bird.
Mewtwo: Unnerve? Give him Pressure! Otherwise good.
Arceus-Water: He can't be Water unless he's holding a Splash Plate. I wouldn't recommend him. I'd say Electric will do you better, as it will scare away Waters and has but one weakness. Otherwise, good. (And Hyper Voice wouldn't be STAB either.)
Entei: Bad. Not your set, but just needs to be replaced. How about Rayquaza as a lead? It will destroy other weather leads like Kyogre and then Groudon can come in. AIR LOCK!!
GIve it sweeping moves, I prefer ScarfRage, but it's your choice.