Youre obviously a first time pokemoner...if that's a word, so I'll break down some fundimental flaws of your team before I post a rating.
First, it's essential that you balance your teams level. There is really no point of have a lvl 44 larvatar when you could just use your mewtwo and it would get any job done better. Get them all to an even level, it aint that difficult (i have three teams all at level 100, no rare candies)
Also, we have a real issue with type balance as well. Pick one, articuno, zapdos or charizard, THERE IS NO POINT IN HAVING THREE FLYING TYPES. Lets just say i happen to have a lvl 65 Golem on my team with rock blast. That easily eliminates your charizard and your articuno (both with a double weakness to rock, fire, ice and flying are all weak against it) and most likely your zapdos also (although only a single weakness). You also have two Psychic types which I have no idea why as psychic is one of the worst types in the game (only two advantages, poison and fighting. Fighting itself has five, rock, steel, dark, ice, and normal).
Now I'll analyze the individuals. Articuno, Fly is a good choice as he doesnt learn any good flying attacks on his own. Mind Reader could be better, but I'm not really a fan so keep it if you want. The real issue is blizzard and ice beam. A lot of first time players have this issue, so dont feel too bad, but think about this. Why would you ever need two attacks of the same type and different strengths/accuracies? If youre fighting a Rapidash and you use icebeam on it, it wont be very effective. What makes you think blizzard will, its the same attack with more base power. Consider this for a second. Eliminate blizzard or icebeam and replace it with a personal favorite of mine steel wing. With this attack, you can use Articuno to fight off its double weakness to rock. Now I realize that Ice is super against ground and most rock types are ground too, but just to be safe, i'd get rid of the chance.
Zapdos: overall not bad. I really dislike charge because you could just use thunder twice and it would have the same effect, but to each his own...Thunder and drill peck magnificent choices and return is iffy as Hyper beam is much more powerful and the same type, but good work on this one
Slowbro: getting there. bravo psychic, bravo surf. I can tell you used him to move boulders outside of battle, but I assume you've beaten the league now and have no reason to use strength anymore. Take his to Fushia, delete strength and get hyperbeam or better yet ice beam for grass types. Also, rock smash is good as its a fighting type and that beats dark which beat psychic, but it's a really weak attack. You'd be better off with brick break (although im not sure he can learn it) but way to mix up the types of attacks here.
Charizard: Ugh...Same issue as articuno except three attacks instead of two that are the same type. You don't need three fire atacks, more like one. Dragon Claw is good, especially against Lance, but you realy could use a flying type attack to take advantage of his second type (fly is a good choice...) and maybe something to help against rock types (double weakness)
Mewtwo: again, not crazy about mewtwo and slowbro on the same team, but i can tell you really like mewtwo and a little repeating isnt that bad. Psychic yes, recover yes, safeguard i can live with, swift no. There are WAY better moves for mewtwo and dont even get me started. Look around, don't be afraid to use TMs.
Larvatar: I can tell he's still in the process, but let me point a few things out. Sandstorm will become unnessecary(sp?) as Tyranitar's ability is sandstream which will whip one up automatically. Rockslide and Crunch are a yes, and thrash is good for now, just be sure to replace it with HyperBeam.
Final rating: 4.5 but excellent for a beginer. A few easy fixes with movesets and stuff (TM 15 is your friend) and you'll be up to a 7. If you disagree or are really ****** at me, forget it, i'm sorry, just trying to help out. Good luck!!