Dratini by far. I knew what it evolved into, and I was determined to get it there. After lots of training, I finally got it to evolve before the E4, and wow was it good. I used it on almost every pokemon, and it didn't let me down. Though it took a while, leveling Dratini was fun for me, as I was just dreaming of the time when it would evolve to Dragonite.