I have a hard time judging one over the others, but "Ender's Game" was my first taste of the Ender series, so it's a bit more special for that reason. You have to read them all ... and they should be assessed as a complete series, IMHO, not so much as separate entities.
Also: The Foundation series by Isaac Asimov ... or anything by "The Good Doctor" ... the first sci-fi book I ever read was "Pebble In The Sky" ... I was hooked for life!
I think you have to segment them..Orson Scott Card, despite being my second favorite author..he has a problem with overdoing successful things...he gets kind of greedy. Ender's Game and Speaker for the Dead were released as a omnibus called "Ender's War"
Xenocide was left out, even in future releases (Ender's War was released in 1986, Xenocide was released in 1991) because it was just not really a good book. Really, it wasn't...it was his "Godfather 3".
Ender's Shadow was fantastic. Shadow of the Hegemon was amazing. Shadow Puppets...was not. Shadow of the Giant was a little better, but meh all the same.
Seventh Son, Red Prophet, Prentice Alvin, and even Alvin Journeyman were all fantastic books. Just all amazing novels. Heartfire, was not.
Ender's Game will always be my favorite, but Speaker For the Dead is the better book...he really hit his stride there, and the fact that Speaker came out a year after Ender shows you that his head was boiling with ideas waiting to get out.
Funny, I can easily name my favorite author, (Isaac Asimov), as his writing was what opened my mind to a complete new dimension and the realms of boundless imagination, but naming my "second-favorite" author would be nigh impossible, and would have to be a group, and not an individual.
Card would easily be one of that group, but also Arthur C. Clarke, Gregory Benford, Ben Bova, Greg Egan, Frederick Pohl, Connie Willis, Kim Stanley Robinson, John Varley, Harlan Ellison, Jack Vance, Gordon R. Dickson, Brian Aldiss, Larry Niven, Kathe Koja, Steven Utley, James Patrick Kelly, Robert Reed, Ian McDonald, Karen Joy Fowler, Walter Jon Williams ... man, I could go on and on, and I don't think I could pick one or two above the rest.
So much amazing literature in this world, and this is only sci-fi I'm touching on! Very few genres that I don't enjoy, but science fiction is easily my favorite. For those thinking about "testing the waters" of science fiction, short-stories are innumerable and a great place to start.
My favorite stories by far: "Press Enter" by John Varley, "With Virgil Oddum At The East Pole" by Harlan Ellison, "The Jaguar Hunter" by Lucius Shepard, and "24 Views Of Mount Fuji, By Hokusai" by Roger Zelazny ... these are four of the most amazing stories I've ever read, and you won't need to sacrifice a huge block of time.