Deoxys. Love the look, and the idea of a Pokemon that mutated from a space virus is pretty rad.
As others have said the strongest is likely Arceus.
I think they could do with a few less new legendaries per game. One trio, the two cover legendaries, and one or two event Pokemon seems like a solid formula to me. I don't mind them putting legendaries from older generations in newer games, though.
Mew question has been answered already.