It depends on what you're looking for. If you're looking for cash for a skill like herblore, go and kill green dragons. If you're going after charms, go to waterfiends (or skeletal wyverns, for a chance at granite legs AND a visage). Killing monsters for effigies should be a secondary goal. Your main goal should be to kill things for money or raw materials for the skill you are planning to use the dragonkin lamp on. Also, I find the effigy drop rate to be 1 per 3-4 hours at waterfiends, from personal experience getting 93-96 summon there. For wyverns I also got a 1 every 3 hour record there as well.