As of the first Breakout wave, Furno has been released as a constraction figure more times than any character from BIONICLE. However, that's not counting the BIONICLE sets with minifigures. In that regard, I believe Hewkii was the winner with ten appearances:

1. Huki (2001)

2. Hewkii (2003)

3. Toa Hewkii (2006)

4. Lava Chamber Gate (as a minifigure, 2006)

5. Piraka Stronghold (as a minifigure, 2006)

6. Race for the Mask of Life (as a minifigure, 2006)

7. Toa Hewkii Mahri (2007)

8. Barraki Deepsea Patrol (as a minifigure, 2007)

9. Toa Undersea Attack (as a minifigure, wearing a Volitak for some reason, 2007)

10. Toa Terrain Crawler (as a minifigure, 2007)

Taking that into consideration, Furno has a ways to go before he dethronres the true King of Being Released Over And Over Again.