I have to use honest words while rating teams. And with this in mind, I am sorry to say the harsh truth but your team is not viable at all in the Ubers metagame.
It is fine, though, as you're still new to this game which got a new complex dimension. Competitive Pokémon is easy to learn but tough to master, and while I am no expert, I can see definite flaws in your current team.
Let's start with the obvious. You're running too many Mega Pokémon, where only one can be used. A common argument that is brought up insists on the versatility of this: you get to Mega-Evolve the one Pokémon that's most viable as a Mega in the current game. However, what you may ignore is that Aggron, without its Mega is completely outclassed by Ferrothorn in the Ubers metagame. Mewtwo's physical attack set, without mega-evolving, is underperforming. Charizard lacks the stats to leave its mark among those big Legendaries who will shrug the hits off and move on. When you have bulk monsters such as Giratina and Arceus, Charizard's damage output is laughable.
Another flaw is that you're using underperforming movesets on some Pokémon, such as Charizard and Deoxys. I'll go into details in a few lines.
I suggest you to play Random Battles first to get a grasp of different Pokémon of all kinds. I'll equally need to butcher your current team into a much improved version, that has to include stronger Pokémon and better movesets. Are you okay? Here's my revised team, that keeps in mind your original build, only improved.
Latios @ Soul Dew
Ability: Levitate
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Draco Meteor
- Psyshock
- Grass Knot
- Thunder
I replaced your Charizard with this. Soul Dew is a godsend for Latios, and this moveset in particular aims at eliminating Groudon-Primal and Kyogre-Primal with most ease. Latios is relatively easy to use and thanks to his superior stats, he can fire off mega powerful Draco Meteors where Charizard would fail due to lower stats.
Next up: Deoxys.
Deoxys-Speed @ Focus Sash
Ability: Pressure
EVs: 248 HP / 244 Def / 16 Spe
Timid Nature
- Taunt
- Stealth Rock
- Extreme Speed
- Magic Coat
Setup Stealth Rock first then play accordingly to your opponent. For example, should they lead with Darkrai, they'll probably attempt sleeping your Deoxys with Dark Void and setup on it with Nasty Plot. Instead, use Magic Coat to bounce the sleep right back to them. If you find it too difficult to play, switch to Deoxys-A until you get a better understanding of the game's mechanics.
Deoxys-Attack @ Life Orb
Ability: Pressure
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Psycho Boost
- Low Kick
- Ice Beam
- Extreme Speed
Alternatively, you can use Arceus in order to avoid having two Psychic-type Pokémon in a metagame where Dark is everywhere:
Arceus @ Life Orb
Ability: Multitype
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Swords Dance
- Extreme Speed
- Shadow Claw
- Earthquake
Next up: Aggron.
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 4 Atk / 252 SpD
Sassy Nature
- Thunder Wave
- Gyro Ball
- Leech Seed
- Protect
I had to replace it. You get to keep Mewtwo, and Ferrothorn offers useful resistances to Xerneas, Latios, Latias and common Dragon-types in the tier.
Rayquaza @ Life Orb
Ability: Air Lock
EVs: 252 Atk / 40 SpA / 216 Spe
Naive Nature
- Dragon Ascent
- Draco Meteor
- V-create
- Extreme Speed
The Speed EVS and nature are designed so you retain most of the speed you actually need. The rest is put all in Attack mainly and a little bit in Special Attack in a way that Draco Meteor still hits quite hard. This is an excellent offensive Pokemon.
Thundurus @ Life Orb
Ability: Prankster
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Thunder Wave
- Thunderbolt
- Taunt
- Grass Knot
I replaced Jolteon with Thundurus so you get a speedy Thunder Wave user. With Taunt, he can screw up defensive / setup Pokemon your enemy may use. He's simply stronger than Jolteon in all aspects anyways.
Finally, your Mewtwo set is quite acceptable. The only change I'd make is move HP EVs into Speed directly because you'd love to move first.
And that's your revised team. Far from being a perfect team, but a very decent first team to experiement Ubers until you can get into more complex strategies. Have fun!
