This might be a broken record for most people, but I think that at this point, Raichu, Dragonite and Flygon deserve it the most, for quite obvious reasons: most other memorable Pokemon from their generations received them but they didn't, particularly Flygon and Dragonite who are basically the two remaining psuedo-legendaries from Gens I-III who didn't get a mega. Raichu has not been treated well by GF, and while Pikachu is their mascot, I think its entire evolution tree should receive love and remain relevant in the series, if anything it only makes Pikachu stand out more when its evo/pre-evo get treated right.
After those three, I would consider Farfetch'd. While an actual evolution would be better for it, it doesn't seem like we do those too much anymore (cross gens, except Eevee). It's one of these joke Pokemon from Gen I, except he has no evolution to redeem it, and honestly I think it could be a better Hawlucha with the right (mega) evolution and its awesome stick technique.
Next, Butterfree. Kind of uncool that its brother from another mother received a mega but it didn't. It could be quite unique and have a Bug/Psychic type, with the same treatment for its Special stats that M-Beedrill received for its attack and speed. The design could be glorious as well.
That's basically it for me. I would love it if Gen V as a whole received the mega treatment more, especially the earlier-areas Pokemon like Stoutland and the elemental monkeys, and the psuedo legendaries. Other than that, it would be nice for the remaining starters to receive a mega, and for every new trio to receive their mega right then and there when they're first released. It kind of bummed me out when Chesnaught, Greninja and Delphox could not mega evolve when the first generation starters could in XY. It would also force GF to plan ahead and make the new starters useful (for different purposes) with or without their mega evolutions, so they won't be too underpowered (or too overpowered) without (with) their mega evolution.
