don't know i guess BOTH because they kinda both have the same strength as each other

So far, Mewtwo is able to own every single Pokemon. Proof? Well, I will give my "proof."

Now, let us look at what each major legendary possesses;

Arceus : Meteor level, got owned by "city-sized" meteor and lost all his plates, got defeated by Pokemon and cement, even with rage not able to break free, and oh, for a Pokemon who created Sinnoh, he sucks, he can't even destroy a "ruins" with his signature move, "Judgment." Even with full plates not powerful enough to withstand a "too powerful to block / absorb" meteor! Arceus isn't even smart in any way, not knowing even the effects of cement, which is created by "materials" he provided during his creation of Sinnoh...Pitiful!

Deoxys : Able to change forms at will; died for 4 whole years to one Hyper Beam from Rayquaza, not even able to defeat other Pokemon with copies; fragile; crystal able to be broken by laser in movie, cannot even destroy an area of ice, even in Attack Forme attack did not kill Rayquaza in two or three hits.

Dialga : Not able to stop time, as proven in last three movies, owned by Arceus who can't even destroy "ruins" as mentioned before, owned by Giratina, tied with Palkia, only able to send people back in time, but not itself...

Palkia : Not as strong as Giratina, Arceus with Psychic attacks broke through Palkia's space "trap," able to shield moves but shield is useless even to planetary damage, "Judgment"

Giratina : Predictible with Shadow Force, Aura Sphere did little damage, even if supereffective to Dialga....slow movement...moves easily dodged by Arceus, what else can I say?

Regigigas : Extreme noob. Pushing a glacier with help from Mamoswines; he's supposed to join "continents" together, but failed...no more to say...

Rayquaza : Not immune to dense and reckless in attacking, double weakness to ice, able to survive in ozone pressure..nothing so special

Now for Mewtwo.....

Mewtwo : Higher IQ than even Alakazam, has access to devices, a strategist than even boasts a plan to rule the world, planet-destroying storm, able to create a hurricane to destroy the entire Earth, according to manga, claimed the most powerful Pokemon, able to relfect, not deflect, moves, creating a barrier to block any technique, and also, a very broken ability, the power to cancel any special abilities in battle, which could immediately dispatch Arceus, because without Multitype, Arceus even winced at Pikachu's thunderbolt, claimed by Blaine in the manga to be able to "adapt" to any fighting style, able to create this;

Planet-leveling Hurricane > City-sized Meteor

which could immediately defeat Arceus oh so easily. A storm has no element, and Mewtwo could teleport cement to his arsenal and drench the "clueless silver water" on Arceus, able to deflect Deoxys' moves, shrugged off Mew's mirror coat attack from his Shadow Ball, has DNA of Mew, who has every DNA of all Pokemon, and yeah, able to destroy an entire island without even half-full of power, and also teleported an entire canyon, which is about as big as Mt.Everest, or even bigger, probably, and didn't even be exhausted after his "moving house" thing, and more of that. He erased memories and also reversed time, as shown in Movie 1, how Ash and other forgot everything and also they landed up in the harbor dock, and oh, watch the Japanese uncut version for Movie 1, then you'll see how powerful and overpowered he is compared to other Pokemon.

You probably meant Ben 10's cousin who is

Gwendolyn "Gwen" Tennyson.

The "Ben 10" character Gwen Tennyson has been played by Meagan Smith - Ben 10 (10 years old) , Haley Ramm - Ben 10: Race Against Time , Ashley Johnson - Ben 10: Alien Force and Ben 10: Ultimate Alien (15 and 16 years old) , Tara Strong - Ben 10 (30 and 42 year old in "Ben 10,000" and "Ken 10") and

Galadriel Stineman - Ben 10: Alien Swarm .