I think that putts with subtle breaks are the hardest to read. I also think that matching the read, line and speed is very difficult for 18 holes. Some days I just "see" lines better. I do know I always hit a better putt if I make up my mind fairly quickly and don't change my read when I get up over a putt.
Here's how I do it. I look at the putt from the side first to determine how much uphill or downhill. So I set the speed first in my mind. Then I look at the putt from whatever is lower, behind the ball or behind the hole. If I'm still confused, I'll look at from the other direction.
I "see" a line that I think the putt should travel on just like you would if you were putting on dew covered greens. When I get over my ball I trace that "dew" line from my ball to the hole, then from the hole to my ball, then fire away.
Sometimes when I can't make up my mind if a putt breaks 12" or say 16", I visualize where would my ball end up if I putted my ball straight at the hole. Ok, it would fall 12" below the hole, that's how much loop I would play.