I began slaying at 75 combat all the way up to 130. From then on I only bothered to train summoning and prayer. The 130s/late 120s are the hardest, because you're pressured to have the best equipments and skills (94 magic, 95 prayer, 96 summoning and herblore) You're constantly at bosses trying to get more. Even then, there's still better armor to get that requires more time invested (divine, etc). Finally, when you get everything, you can run around killing random people in pvp worlds with a VLS or some other overpowered weapon.