I keep asking myself that same question...what is it that makes a daylily a good Bloomer?
Of course the geographic area you live in will be a determining factor in what plants will be your best bloomer. I prefer the term good "garden performer". There are several threads here in the forum that address garden performance and many people list what their best ones are. There are traits that are often not thought of that can contribute to the plant being a good garden performer. Of course what we are normally looking for is a lot of blooms and a long bloom cycle.
There are so many traits that will contribute to those goals. Too, even though the question specifically asked about the best bloomers, who wants a plant with a beautiful bloom and embarrassing foliage? So a great "garden plant" will have good foliage along with lots of blooms over a long period of time. The plant will have disease resistant and resistance to pests.So once again your geographic area will have a great deal to do with the diseases and pests you have to deal with. As was stated above by others, the blooms need to be presented in such a way that they are above the foliage, and not so crowded they prevent each other from opening fully. The scapes have to be tall enough to display the booms, strong enough to support them without leaning or breaking. The scapes have to be well branched, etc. Rust and leaf streak can distract from even the most beautiful blooms. Thrips, midges, slugs, and snails can devastate the foliage and the blooms. So being resistant to all the above are certainly traits that would help make a plant a good garden performer.
It is not just a choice of one plant over another, but the selection of plants that bloom at different times of the season: Usually referred to a early, midseason and late etc. If you have very early blooming plants that open when the weather is cool you may find they don't fully open correctly, so that is something to take into consideration, you might want cold morning opening plants, early morning opening plants.You might want extended bloom from your plants so they are open when you return home from work. For the plants that bloom later in the season you will want blooms that can withstand the heat and not fade or "melt" in the hot sun.
The things we normally think about of course are bud count and branching along with rebloom when we think about a large number of blooms. I am currently trying to grow more "modern" daylilies with the higher bud counts and with a higher number of branches. However, 'Filled With Joy' one of the best performing plants in my garden this year is registered as three way branching and a bud count of 10-15 buds.What this plant lacks in bud count and branching it has made up for with its ability to rebloom in my garden.
The list of things that determine the best bloomers in the garden goes on and on, the choices vary from garden to garden. I posted a thread recently (I have received no replies so far...hint hint) asking about plants with a high branch count and the relationship with reblooming because I don't have much experience with these plants. Just so many things to consider, but that is what makes growing daylilies so interesting.
So I certainly suggest reading all the threads about "your best garden performers", "reblooms" etc. Some of these go back a few years and are full of wonderful suggestions.
