Kathryn's Digital Designs
Kristin Aagard Designs
This, I think, is a good start and I am sure the Bug can add quite a few of her favorites, which won't be anything like mine. Take your time when checking out the designers. Find the ones that jump out at you and say "I'm yours". These are going to be the ones you use time and time again. Some designers specialize in elegant pieces that are perfect for weddings, anniversaries and formal events, as Verena Karolyi and Laitha. Others are more playful such as Kathryn's Digital Designs and Kristin Aagard Designs. Colie's Corner kits are almost always workable for boys (and those are hard to find). Linda, Connie, Kristmess Designs and Captivated Visions kits are well rounded and they produce them for a variety of categories.
Another point to consider when you are downloading freebies and/or purchasing kits: Unlike paper scrapping, digital kits can be used over and over again. I prefer to make purchases from my favorite designers knowing that I am going to get a quality product everytime. And if something is wrong, they are going to fix it.