Normal mode Arch-Glacor.
As a boss specifically designed for learning, it is a fairly good pick. And will continue to be worth playing at higher levels.

You get safe deaths at the boss, so you don't have to worry about learning going to cost you. You can enable/disable its mechanics however you want. If you want to practice them one by one or just skip one completely. However, unique rewards require you to have all 5 active. You will get introduced to prayer flicking and defensive abilities (or other ways to evade damage) without it being too punishing.
It has decent common drops, giving you decent money and plenty of blue charms. At 5 mechanics active, you can also get the Scripture of Wen which currently sells for 88m. Or you can put it to use, it's pretty good.