For me, the best thing is meeting all these characters I've created and watching them play around in their worlds. I love getting to know these unique people with their own wants and needs and fears, putting them in situations, and watching them try to get themselves out of it. I love crying with them when they're hurt. I love the sense of pride and joy I get when they triumph over whatever adversity plagued them.
Nothing is better than watching your children (real or imaginary) grow and change right before your eyes.