Okay, guys, who deserves it more? Ridley or Mewtwo?
That's a rather tough question.
On one hand, Mewtwo is one of the most popular and iconic Pokemon of all time, and Pokemon itself is an absolutely colossal series; it's presence is phenomenal, and Mewtwo is an important part of that. He is definitely a Nintendo All-Star, and as such is an ideal choice for the Smash roster. What complicates this further is the fact that his importance was recognized once already; he was a playable character in Super Smash Bros. Melee. Yet despite his importance, he was cut in Brawl, to the dismay of many. Nowadays, Mewtwo is even more prominent and more popular than he ever was before, and is one of the most requested characters for Smash. The timing is absolutely perfect. So, you could say that Mewtwo is the more deserving of the two due to being an extremely prominent Pokemon character, and because he was unfairly cut after he was made playable in a previous title, and thus deserves to return.
On the other hand, you have Ridley. Ridley is the main antagonist of one of Nintendo's biggest series, Metroid. Like Mewtwo, Ridley's importance to his respective series, and the importance of said series to Nintendo as a whole, makes him a Nintendo All-Star. In addition, Metroid has only ever received one real character in Smash, being the main character, Samus. In Brawl, rather than getting an entirely new character, Zero Suit Samus was added as a newcomer. So, you could argue that Metroid is deserving of a truly new representative. Also, unlike Mewtwo, Ridley has never been a playable character in Smash before, yet has been one of the most desired characters ever since the beginning. Despite being put into non-playable roles in previous titles, and despite the many detractors Ridley has, support and demand for Ridley continues to grow exponentially. So it could be argued that Ridley is more deserving due to the lack of representation his (important) series has received in Smash, and because he has never had the chance to shine in Smash before.
Really, I think it comes down to personal opinion. I, for one, think they both deserve to be in. I don't think there's a good reason to exclude either of them.