There's a very long list of what's frustrating about painting portraits. The first being that people don't expect to receive a painting of a cute baby, they expect it to look like THEIR cute baby. That last phase of editing can be excruciating for me. When I realized that I needed to scrape the paint off this little one's eyes and paint them a few millimeters lower, I wanted to cry. But... it that's just part of the process.
(I think next I'll paint some nice autumn leaves... no one minds if a leaf vein is just a little off or a shape is just a little skewed when it's a leaf!)
But, back to the original question...What's the most fun? There's always a "Oh, there you are!" moment for me. I'm just working along with colors, shapes, tones, and shades when all of a sudden... It's a little one with a personality looking back at me! I love that moment! Sometimes it happens when I put the little fleck of white sparkle in the eye. Sometimes it's when the dimple is finally right or the highlight on the end of the nose is put on. I usually have a "Well, hello there!" moment right then.