I can't pick a single favorite. Perhaps I could say my favorite is the one I just bought! Like everyone else, I love the collabs because they are so versatile. Generally I prefer blues, and since my family is full of guys, I use a lot of kits with blue. For young kids and sometimes older ones or even adults, I sometimes like bright colors. And since I am doing a lot of heritage pages and my own story, I use a lot of neutral color kits - just did one with SweetMade's My Shabby May Flower
It's a pleasure when I have the opportunity to use a more feminine kit and have used Andrea Gold's Just Because
At this point I have LOTS of kits, so when I buy, I'm looking for unique elements that fit a photo or photos I want to scrap. I do often pick the kit first and then hunt for a photo. Since I have many thousands of photos, I can always find something that works. And I almost never use more than one kit (or bundle - I do love those!) for a single layout. My stash isn't well enough organized to easily find coordinating stuff from different kits. If colors in the photo or photos don't go with the kit, the photos are turned into B&W or sepia.