Default blizzard UI.

It is important that your UI has all the necessary information arranged in a way that makes the most sense to you.

I heal using party1/2 macros with shift and ctrl modifiers. I use mouseover macros for party 3 and 4. This is what I find most efficient for me; using healing addons means that I have to mouseover a frame, then make sure it is the correct frame, then press a button. Too much to concentrate on when I could just press a modifier macro and know for sure that a spell will be casted on party1/2.

You should heal using what feels most comfortable to you.

It came to be this way because I used to be a pvp player since wotlk 2010.