Greetings, Club. Long time no see...
On the current Topic: I'll go with Mewtwo's Background Story because it is just all around EPIC. Mewtwo and Mew duel it out, I mean, come on, how can that not be awesome? In fact, about all of it is really Mewtwo's Story-line! The cloning of the other Pokemon in Clone Balls. How it was created. Not to mention that Mewtwo's Story is read/seen in anyway possible, even in it's Comic Book issue (Which is VERY short for a movie story in my opinion). It is my favourite legendary, and it'll always be the strongest legendary in my opinion, even if they make a stronger Pokemon. But I'm not saying that it's the strongest Pokemon ever. Anyway, here is Mewtwo's history (In a nutshell) with information from Bulbapedia.
This mentions some things that probably has not been mentioned in Mewtwo Strikes Back or Mewtwo Returns. In fact, apparently, this Extra Information came straight from Pokemon Games, which appeared in the Mansion of Cinnabar City, which can be accessed in Pokemon Red, Blue, Green, Yellow, Fire Red and Leaf Green Versions. Cinnabar does appear in Pokemon Gold, Silver, Crystal, Heart Gold and Soul Silver but the Mansion had been swept along with Mewtwo's history, leaving the information inaccessible. But you would still know most of the information anyway, wouldn't you?
By the way, the information states that Mewtwo is encountered in Cerulean Cave (Not in Pokemon Silver/Gold/Crystal), in which the evidence suggests about Mewtwo leaving New Island in Mewtwo Strikes Back, and therefore could have left for Cerulean Cave.
Mewtwo was actually, in the evidence, said to have been born of a Mew, which was ridiculously PREGNANT (Which is UNBELIEVABLE, as Legendaries should never breed, even if a legendary does). THEN all the experimenting happened, with all the Scientists plus Blaine and Dr. Fuji and so on. Other than all those things, I don't think there is much more to say about Mewtwo's Origin, why I like Mewtwo, or about Mewtwo Strikes Back. We probably already said it all out considering there isn't a lot to point out anymore.