Well, first off, Hot Toys does 1/4 (18") and 1/6 (12") scales, they don't do 6 inch.
Mezco was the first company to really attempt to do what Hot Toys does - really-high-end figures at a high price tag with real clothing - at the 6" scale. There are of course other companies who work in that scale as well - some high-end ones, like Mafex, too, but many of those focus on articulation and design rather than that "close to lifelike" concept. For me, at least, being all-plastic is the difference between a high-end figure and...well, not, but that's just me
I agree with Grimlock. It's much easier to pick a criteria and stick with it, whether it be a certain license (read: Star Wars, Marvel, etc) or a certain scale, or a particular line (say, Mezco's One:12 Collective). If you try to get too much, too fast, you'll burn yourself out quickly, and end up with a bunch of stuff you kinda like but don't love. Take it from someone who knows. I personally picked certain licenses (Star Trek, MCU, James Bond, and LOTR) in the 12" scale. And that's plenty to keep me occupied.
I also wouldn't recommend being a completist. That's wayyyyy to expensive. Especially in 6" - it's a difficult scale, because there's just such a variety there. You've got mass-release kid's toys, to Hasbro's Black Label, to Mafex - and they're all trying to hit a different price point and a different type of collector. Most of the 12" scale companies are all trying to make Hot Toys-like "realistic" figures, so the collection looks more cohesive, I guess.