A lot depends on whether your interests are limited to current (or recent) production knives, or knives from other eras. The time period from late 1800’s to WWII is generally referred to as the Golden Age of American cutlery. A time during which there were literally hundreds of cutleries in operation, many making and selling under their own brands, but also making private label knives for hardware distributors. During that time quality was a given - it was the ticket into the game and if you didn’t have it you didn’t stay in business long.
It’s a fascinating, wide collector focus having too many different options to list in a post, but this forum is filled with posts, pictures, and information about those companies and their knives. There are many sources for finding such knives, including but certainly not limited to the AAPK stores here. Like you I started out collecting Case, then branched into other brands.
Probably as good an answer to your question as any can be found here https://www.allaboutpocketknives.com/knife_forum/
in the index to AAPK content. Scrolling down through the Forum home page content will show you the more popular brands (the number of posts contained in each subject subforum gives some indication of the popularity).