Personally I think UA has the best boarding process for my preferred boarding style (which is boarding toward the middle or end of boarding instead of the beginning unless I have a carry on that has to be in the overhead).

That's purely due to the fact that UA has done a very good job of segmenting out the Group 1 and Group 2 lines and keeping those lines clear once they've boarded those groups so it's very easy to walk right up to Group 1 when you are ready to leave the lounge and get straight on the plane. They also do a good job of holding the other groups so folks in Group 1 and Group 2 can come through

I find at a lot of places the SkyPriority line gets jammed up once they get through F/Sky which is a problem and that many Gate Agents don't hold the regular line for SkyPriority which means you have to be that person who jumps in front of the next person in the non-SP line instead of having the Gate Agent direct traffic