This is just my experience too - you may build up your own favourites for different reasons.
Far Far Hills http://farfarhill.blogspot.com.au/
Whatever I dislike about Far Far Hills website (mainly the search, naming of kits and ability to find stuff, and the limited and slow download plug in), those kits I've downloaded for free from Far Far Hills are items I use over and over and over again. Particularly their background papers, but also alot of elements. I think on two occasions I have also bought kits from FFH but mostly it took patience in downloading free stuff. It was invaluable in giving me a good range of papers and I continue to go there every couple of months. The items are reasonable quality though most have a texture on them.
|New Clothes 4 Nan - an early page of mine with early old-school kit materials|
I've got some of my most memorable kits from Pickleberrypop. Easy to view, easy to search, easy to purchase and reasonably easy to download. The kits I seem to use the most often from there are Feli Designs, Lara's Digital World, Prelestnaya, Laura Burger Designs, Lorie M Designs, Armina Designs, Jasmin Olya Designs (mainly commercial use flowers), Fayette Designs and more new school Tiramisu Designs, Indigo Designs, and Vero Designs to name a few. They also allow me to save a wish list and my cart for months on end.
|An early page of mine with PickleberryPop kits|
Sweet Shoppe Designs http://www.sweetshoppedesigns.com/
Where Far Far Hills and Pickleberrypop are my meat and potato sites, sometimes I need to go further afar for eye candy and more new-school products, and Sweet Shoppe is one of those for me. I love the Bad Sewing Machine line from there, the flairs and paint splatters which I have used over and over and over again on layouts. I have also bought 3 kits from there and haven't regretted that either. In total I've bought 11 products from there - and all bar one I use almost one item in every layout I do from Sweet Shoppe!
|One of my many pages with Bad Sewing Machine stitching on it|
Sugar Hill & Co https://www.sugarhillco.com/shop/home.php
Another eye candy site, where I tend to purchase jewels, plastic looking items, buttons, very natural flowers and sprigs, ribbons, pleated trims, and things I just can't get anywhere else. Again, items I've purchased from Sugar Hill & Co I have a high usage rate on. But I haven't bought any full kits from there.
The Digi Chick http://www.thedigichick.com/shop/
A friend recommended this site to me, and I can see why. Here I found kits and elements that I wish I could find years ago. Exciting, good quality, and alot of great gap fillers. I also bought my first lettering kits from here - ones that I just can't make with fonts alone and I use them over and over.
Frou Frou Craft https://creativemarket.com/froufroucraft/shop
This is one of those sites operating out of an Etsy-like environment; hand drawn / painted pieces by an artist rather than made up of commercial elements or traditional scrapbooking kits. And though its very specific and will only appeal to a small audience, there are more and more sites popping up like this recently. I really like some of papers from this site and those I bought I use and love love love them!
|A page with Frou Frou Craft patterned paper, and collection of Sugar Hill buttons|
Cilenia Curtis - Scrap Art Studio
I got onto this site way too late, and it has since closed. I bought a number of kits from there that seriously filled some gaps. Staples, tags, grungy, paintery effects that I still use items over and over and over again.
|A page using mostly items from Cilenia Curtis|
Those I wish I hadn't bought?
Of course, there are always some kits I purchased and I regretted it. All I can recommend is 1) never purchase kits when you're in a foul mood or when you feel like you need retail therapy 2) think about five ways you can use that kit rather than just one 3) put it down to experience - its bound to happen eventually 4) delete the kit if you never use it - its liberating and saves room 5) try to look at the kit as a whole - is it really worth it if you're only buying it for one pretty flower?
Items I wish were more readily available
There are still occasions where I look for some items in vain, and either make my own or scan my own in order to fill the need/gap.
Fairies, toadstools / mushrooms that aren't cartooned with smiley faces, glass items such as vases or glasses (that don't look cheap and nasty), silverware ie knives and forks!, animals that don't look like a 5 year old drew them, satin ribbon that is just straight, fimo style products that are not flowers, frayed material pieces, burnt edged paper, swirls/flourishes (can never have enough of those), food (even fruit and veg is hard to find), gold items, silhouettes, twigs of flowing / interesting sweeps/shapes, rocks!, digital items like mice, keyboards, etc, pens, pencils, paint brushes, just to name a few!