Yeah, the game's an absolute joke compared to competitive play.
For competitive we do need to know your held items and EV spreads, and we assume flawless IVs unless either otherwise mentioned or in the case of Hidden Power, in which case we assume the best possible IV spread for that Hidden Power type.
With what you have here:
Hydreigon's a bit of an unusual set, mixed Hydreigon typically look like:
Hydreigon @ Life Orb/Expert Belt
EVs: 4 HP / 252 SpA / 252 Spe
- Draco Meteor
- Fire Blast
- Roost / Earthquake / Dark Pulse
The reason for this is that Hydreigon has insane coverage and great mixed offensive stats. Draco Meteor usually means something's going to die. Fire Blast hits the Steels that can take a Draco Meteor. Superpower can break down common Special sponges like Blissey and Tyranitar. For the last slot, you want Roost if you're using a Life Orb, definitely. If you go for Expert Belt, one of the coverage moves is probably a better choice.
Venusaur shouldn't be used outside the sun. I'm going to suggest a Ferrothorn here.
Ferrothorn @ Leftovers
Trait: Iron Barbs
EVs: 252 HP / 88 Def / 168 SpD
- Leech Seed
- Thunder Wave
- Stealth Rock
- Power Whip
Ferrothorn is amazing and I've almost never made an OU team without it. Even when I do skip out on it, it's only for Amoonguss. Leech Seed is for recovery and forcing switches, which goes very well with Thunder Wave. Thunder Wave cripples set-up sweepers and Scarfers. And everything that isn't Ground, really. Nothing enjoys paralysis. Stealth Rock is quite possibly the best move in the game. Competitive teams that don't run it better have a REALLY good reason they aren't. Power Whip is your attack. 120 Base Power and 85% accuracy with STAB to boot is great. It isn't quite Gyro Ball's 150 Base Power with 100% accuracy, but there are reasons for that. Gyro Ball, first off, is not reliable damage-wise. Second, Thunder Wave is a large part of what makes this Ferrothorn so good utility-wise. Gyro Ball is pretty bad alongside Thunder Wave. You get pathetic Base Power by comparison to Power Whip. Third, Steel has atrocious coverage offensively. Grass hits common Water-types like Jellicent and Gastrodon very nicely.
Braviary is a bad idea in OU. It's outsped by most of the tier, it has a glaring Stealth Rock weakness and... It's just not OU material. Fantastic in NU, where it has literally no hard counters, but Ferrothorn is everywhere in OU and it's Braviary's worst nightmare. I feel like a Skarmory would be pretty good here.
Skarmory @ Leftovers
EVs: 252 HP / 232 Def / 24 Spe
- Brave Bird
Skarmory's extremely useful for a number of reasons. It's one of the best physical walls in the tier, it has reliable recovery, it has access to another hazard you can use in Spikes and it gets Whirlwind. Roost is that recovery I mentioned and it sheds an Electric weakness for the turn if you happen to be faster, which is nice. It does give you a Ground weakness for the turn too though. Spikes is not quite as good as Stealth Rock but it's still great to have. Whirlwind is also really useful, since what you pull in takes damage from whatever hazards you have set. It also phazes, which means nothing's going to set up on you without a Sub. Brave Bird is your attack and despite the defensive spread it still hits pretty hard.
Chandelure, yes. Keep this. The Fire immunity is great (you have several Steels, so it's kinda essential) and it spinblocks at the same time, the reason to use it over Heatran. Spinblocking is important if you're running Skarmory and Ferrothorn, since you'll be stacking hazards quite a bit. But I'd suggest using a Choice Scarf with it. The set's the same as what you have (IF that Hidden Power is Fighting. You want HP Fighting). You'd want an EV spread of 4 HP / 252 SpA / 252 Spe and a Modest Nature to go with that too.
Samurott is, again, bad in OU. You have a ton of Water-types to choose from, but I'd suggest Keldeo.
Keldeo @ Leftovers
Trait: Justified (Useless on Keldeo, but hey, no other options)
EVs: 252 SpA / 4 SpD / 252 Spe
- Calm Mind
- Hydro Pump
- Secret Sword
- Hidden Power Ghost
Keldeo is obscenely strong after a Calm Mind. After 2, it's probably going to sweep. Hydro Pump gets STAB and just destroys things in general. Secret Sword breaks Blissey, hitting its pitiful Def stat with your staggering SpA stat. HP Ghost is here for Jellicent that otherwise walls you, and it deals with Latias too.
Finally, Metagross. The one problem I see here is Hyper Beam, which is a godawful move competitively. You will get set up on and swept if you ever use stuff like Hyper Beam. I'd suggest replacing it with Agility, using a Life Orb on the set and an EV spread of 4 HP / 252 Atk / 252 Spe with a Jolly Nature. Otherwise, good.
"Whether Castform the Weather Pokemon can weather the weather is dependent on whether the weather that it's weathering is the proper weather for it to weather. Whether the opponent can weather the weather is also subject to whether the weather that it's weathering is the right weather to weather. Whether Castform can weather the opponent is once again dictated by the weather and whether that weather is the favorable weather for weathering. But most importantly, you should know whether there will be weather to weather rather than what weather they will be weathering and whether they can weather that weather."
- Smogon's Castform analysis