Just as Jukhy said, it depends on the biuld. But more importantly, it depends on what you want to be doing. Usually the healer class is also used to provide boons. Boons are not just a "cool extra" thing in the game. A player with all boons will be more than twice as strong as a player without boons. Because you only need 1 stat in order to heal (Healing) and the healer builds often have access to boons, most healer builds also provide boons. To effectively apply boons you are going to need some "boon duration", which comes from the concentration stat.
So in order to decide what stats you could replace Harrier's with, you would have to decide what your build will be doing.
If you want to be an all-round support with boons, heals, a bit of damage, and being the general backbone of any group (except for raids because of toughness), you could go for celestial gear for example.
If you only want to provide healing, then magi's is a fine stat, but then you are definitely not getting the most out of your class/build, because you could be doing healing+damage or healing+boons.
If you have access to conditions in your build, then you could replace Harrier's with Seraph or Plaguedoctor's
And if you would want to do healing+(power)damage you could look at Zealot's for example.
The reason behind all this is that you always get more than one stat from your gear, so you can never go for a pure healing build, because you are then wasting at least two of the three stats on your gear!
Hope this helps and if you have questions, feel free to ask!