My first thought would be the sawmill operator NE of Varrock. He had the monopoly on making planks for the longest time. (took a while for other sawmill operators to come in) The GE made planks far more accessible to the public, but someone still had to run them. even if you didn't do the job yourself, your Butler/Servant would still have to have the money to pay to covert logs to planks.
Another option would be Death with the taxes you pay when you die. enough players die with the higher end stuff, and it could add up, even though that mechanic hasn't been in game quite as long as other stuff.
Vorago, Arraxor, and other bosses, at least till players learn their mechanics.
Shame the armour stand dummy isn't an NPC, or I'm certain they'd be the richest!
Kind of a toss-up for me between Death and Pikkupstix.
Death hasn't been around as long, but probably has the largest charges of any NPC in game. Given the risks at extremely top-end PVM, I can see him making a healthy chunk of cash each day.
Since spirit shards have to be bought and are "destroyed" upon the creation of a pouch - there's not much of a market for resale. I know there's a few shops which sell shards, but I reckon his was by far the most popular (Until Lord Amlodd, I see competition there...). Since his base stock has something like 8.1B in spirit shards, he's clearly doing alright!
Death has an absolute monopoly, whereas Pikkupstix has some competition (most of it not too serious) but his services have been around a lot longer. Plus players will actively purchase from Pikkupstix, but avoid using Death's services when they can.
Starting to talk myself into Pikkupstix! If you only do what you can do, you will never be more than you are now. Want to play old holiday events? Click here
Pikkupstix isn't the only seller of Spirit Shards. But I guess his location would have been one of the more convenient to use for a long time, outside of Oog-loog. Wonder if all of the Summoning Suppliers are part of the same organization?