For me, it's Pinball. The game is actually deeper than it looks, and the bonus stage was a nice touch.
Pin Bot is a close second. I've never played the actual table but it looks like the developers did a good job with the sim despite the system's limitations. It also has one of the best Game Over screens I've seen in an NES game. Dare Pin Bot to the pinball challenge!
Rollerball closes out my top three. This was the first of three excellent pinball titles developed by HAL Laboratory (the others being Revenge of the Gator and Kirby's Pinball Land on the Game Boy).
What about you?