I'm with demono...I can certainly recall some kits that went together more smoothly than others, or kits where I successfully tried some new technique, but the one I'm working on at any given time will always be my fave!
BUT, in the spirit of this topic, I'd choose the Tamiya 1/25 Tiger I. I had built it 25 years ago as a kid, and was just amazed at its size and what I thought at the time was incredible and accurate detail. It was just a honey to build, no fit problems, and I remember re-reading the huge instruction manual over and over.
Then, I built the same kit about 5 years ago when it was re-released here in Japan. I was aware of the myriad of accuracy issues, so part of the fun this time around was trying to fix all those problems! No PE or anything else available in 1/25, so I made do with what I could scratch together myself, which included completing the partial interior, with engine.
So, that Tiger I was a trip down memory lane plus a technique-building excercise all in one. And it was still a sweet build! Aside from all the accuracy issues, it's a great kit.
I still have the old Tiger I built all those years ago, and it's great fun to compare the two and see the progress!
Here's a coupla pix of the newer of the two: