fletching, cooking, and firemaking seem to be the easiest. Fm you can get 100k/hour with willows, 150k with maples. Cooking i've never timed but i suppose it would be fast, since you cook prettymuch at the rate you light logs Thieving, at 71, also gets 100k/hour at pyramid plunder, and at 91 i think you get about 200k/hour at knights. I've only had experience with fm and thieving with pyramid plunder, as well as cooking (but i dont time cooking). But judging by the number of 99's, those are the easiest 99's