I say the Gaara vs. Rock Lee. I think it was the only match where at least one of the contestants used their power to the fullest. It was cool watching Lee go all out. But it was sad that his efforts were in vein (SP?).
The Shino vs. Zaku comes in Second. But in the manga only. Because in the anime, the bugs didnt make Zaku's arms blow off. They just crippled them.
I actually thought they were all interesting in their own right. Except the Kankuro fight. That was boring.