I haven't been to all of them, so it's hard to say. I bet the first one was fun since it was the first of it's kind and the first Kong Row was probably pretty cool to see.
There was probably a lot of that "newness" in the second one. And overall Donkey Kong felt like it was still being figured out back then, like there was still major variations in gameplay style.
I'm thinking 2 might have been the best....but after talking to Richie, the next one might be a strong contender.