There is no doubt that Ender is an extremely intelligent character. His intelligence and emotional stability is what got him to Battle School in the first place. What is great about Ender is that he is not only book smart, but he is also incredibly street smart. He picks up on school teachings quickly, but he is also able to see beyond the facade of what is actually being presented to him. Take the following quote, for example:

He held up a limp hand. "See the strings?"

This quote comes from chapter 13 and is said to Valentine during Ender's brief return to Earth. The quote is a complaint by Ender that shows he understands he is nothing more than a puppet to be used and manipulated by the adults in his life. The quote is something very reminiscent of what Peter told him much earlier in the book.

If you need a quote that is more focused on traditional intelligence, then I would look to chapter one. There is a great quote about Ender's school ability.

Miss Pumphrey talked about multiplication. Ender doodled on his desk, drawing contour maps of mountainous islands and then telling his desk to display them in three dimensions from every angle. The teacher would know, of course, that he wasn't paying attention, but she wouldn't bother him. He always knew the answer, even when she thought he wasn't paying attention.

Another great quote comes in chapter five. We are told that Ender benefits from practice, but we are also told that he's smart enough to figure out games and systems well enough to excel at them by doing nothing more than observing.

All he had to do was watch the game and understand how things worked, and then he could use the system, and even excel.