Definitely the bottom lane is easiest to climb up. The bottom lane is usually less contested and there are fewer players there. This means that you can get fed easier and have more kill potential. I also find that the bottom lane is the most predictable in terms of champion picks. You know you're going to go up against the usual marksman + support duo. In the bottom lane, you can play more aggressively without getting caught out. I think the bottom lane is much easier to climb up than the other lanes.