The following is a post from someone who is a rookie competitive battler and breeder, so take what I say with a grain of salt.
As others have mentioned, the following information will help a lot to determine good synergy and quality of a team.
-Natures
-Items (especially if you're using a mega, as your entire team can do)
-EV spread
-Moveset
-Abilities
Now then, a few things to toss in:
-Stealth rocks are pretty frequent in competitive battles. Your Gyarados (especially if not mega) and Salamence will not appreciate the 1/4 health loss upon entry. If you have multiple Pokemon that will get hit hard by rocks (especially if someone is x4 weak to rock, like Talonflame or Volcarona) it's useful to have a rapid spinner or defogger to help clear the field for safe entry. Latios can technically fill this role, but defog is impossible to teach in gen 6 without genning (as it's a gen 4 exclusive HM) and very few Pokemon can learn it through levels or breeding. Useful defoggers include Mandibuzz (perhaps the gold standard non-legendary defogger, as she's pretty bulky too) and Crobat. Rapid spinners are more plentiful, and useful ones include Blastoise, Excadrill, Starmie, and Donphan.
-If you're going by Smogon rules, mind your tiers. A speed-boost Blaziken is considered uber (as in competes with powerful legendaries like Mewtwo, Xerneas, and Zekrom). They're not impossible to defeat, but just a pointer if you decide to go that route. I face a lot of kids that challenge me with teams stuffed with legendaries (I beat a Zekrom with a Venusaur. I was extremely lucky). Sometimes I win, sometimes I don't.
-Weather has been nerfed so it's not as much of an issue, but your Tyranitar will be starting a sandstorm upon each entry (pending you're using a sandstream one). It's not a huge amount of damage (and your Metagross won't care), but something to keep in mind. It may sound like a sin, but you can double up certain types as long as you have good synergy. For example, teams built around rain may have multiple water types to take advantage of abilities like swift swim, rain dish, or hydration. If you want, you can try to bring in a Pokemon that benefits from sandstorms, like Garchomp.
-While not a terrible thing, your team is stuffed with generic Pokemon that your opponents may automatically predict and prepare for. Metagross especially is pretty predictable.
-The team has a fairly large ice weakness. Salamence gets hit x4 and Tyranitar and Latios both get x2 (Gyarados won't appreciate it either, but a mega might shrug it off). Ice attacks are pretty common (to counter dragons, of which you have two) and while Blaziken and Metagross may be your only counters, it's something to remember. It's also the most common hidden power (extremely common in Manetrics with people who either gen or actually understand how to breed for hidden powers. It's a bit of a clusterfuck 0-0).
-Fairies are pretty common these days, and like ice, your team won't appreciate the Klefkis and Sylveons out there. Salamence and Latios will both get wrecked and they can't use their best STABs like draco meteor or dragon claw. Tyranitar and Blaziken are also both weak to fairies (EDIT: Actually, Blaziken get's hit neutral, but you won't be able to use fighting type attacks effectively on fairies). Metagross is a decent counter to most fairies (some Azumarills might laugh off a bullet punch or hammer arm), but if you don't have him to fall back on, it may be tough. Don't underestimate the nerfing of dragons. Moonblasts hurt.
-As others have mentioned, it's good to have a wall. Your team is super offensive, and while it can work, it doesn't hurt to consider some that may not hit for OHKOs every day but provide more supportive roles. Ones that can reliably switch in and land a status, set up screens, or tank some hits. I hear the metagame these days is pretty bulky and stall-heavy.
-Also mentioned by others, your team is super physical if you're using standard movesets. Some of these Pokemon can handle multiple or non-conventional roles (like special Tyranitars and mixed Salamences). Part of the reason why listing movesets is important. Physical walls like Slowbro are perhaps more common than special walls.
Pokemon is a pretty deep metagame. Pros can predict movesets and attacks several turns before they actually happen, and everything can change. As kids we play all-out offensive by just matching types, and boy is it a huge wake-up call to see that's not how it works when facing actual people. Lot's of things to consider, and I'm being driven mad by trying to make my first competitive teams, so stick it out :3
