Simply not possible to answer that question. Just like when I bring the latest batch of blooming orchids in the house and call my wife over. She says [Oh, that is may favorite.] And then when she looks at the next, [That is my favorite.] It turns out most everyone of the 200-300 different orchids happens to be her favorite when it is in bloom. Same for this question, can't even begin to limit to a favorite, not even in varous categories. How is it possible to like a tomato better than a cucumber better than arugula? Each is a totally different, non comparable unique experience. How can I love a day lily more than a true lily, more than an iris?
What are the attributes that draw me to a selection? Now that makes a question that can at least be partially answered. At the practical level, the plant needs to be adaptable enough to grow in my challenging environment. If the first experimental plant does not live and thrive, a second will not likely get purchased. I tend to be drawn toward plants that have vivid color and long bloom period. So reds, blues, bright yellows, deep purples tend to most cach my eye. The unusual color is also an attention grabber. For the shade, contrasts and textures are more important. But really I don't have a set of criteria for selection. Such decisions are usually made by this gardener at the emotional level. The visual stimulus of the plant strikes a chord, and I react by placing the plant in the cart. Simple as that, not one favorite or one group of favorties, but dozens of favorites with the possibility of many dozens more favorites as experience provides that opportunity.