Summoning is the fastest exp per hour of all the skills(if you have charms secondarys and shards set up) prayer would be the second, but also costs loads. Then theres herblore..still costing 70m+. I'd say cooking would be next it costs money, but not a lot. Go with cooking or fletch. Fletch is slower but doesnt lose money and cook is a lot faster but loses some. Summoning requires TONS of charms, I reccomend fletching it takes around two weeks, cooking is a fun one too it's around two weeks also, you end up paying around 1.5m for the skill though, cooking gauntlents is STRONGLY recommended for cooking as it will make you get 99 faster.