So atm I have the waist, feet and just now I got the chain healing ring, so I was wondering, which 2 should be my “default” legendaries, and on which fights is it worth to change out to another legendary?
It would all depend on how much use you can get out of healing rain on any given encounter. For example, a fight like Ursoc, Cenarius or Guarm would, in my opinion, favour the ring and the boots since you always have a stacked group of players of some description making both chain heal and healing rain spells you would use more often. Fights like Dragons and Odyn where your raid is spread and moving for larger parts of the encounter makes getting efficent use out of healing rain more difficult.
As standard I’d use the ring (unless you’re never casting chain heal, in which case it goes without saying it’s not useful) and then switch to the belt or boots depending on the type of encounter, ie how much use you can get out of your healing rain. I hope that clears things up.