There are so many great options!
I've been collecting since around 1980. If we are going old school lead miniatures, I was a Ral Partha guy, but I had plenty of Citadels and Grenadiers...and whatever I could get my grubby hands on, really.
The 90s took me hard into Games Workshop miniatures. I still use lots of them, a mix of metal and plastic. Their metal treants from the early 90s are still the best treants ever made, IMO, and they basically defined orcs and goblins for me. They are now the gold standard for HIP miniatures.
Reaper revolutionized the medium with Bones making massive collections far more affordable, and while I think basic Bones (the white plastic ones) are kinda substandard for small and medium miniatures, Reaper have since really upgraded their plastics, and their designs are phenomenal. I particularly thathow they celebrate their artists, and are willing to let different artists put their own spin on designs. Also, I appreciate the occasional NSFW miniature and Reaper has that covered; how else would I have had the opportunity to drybrush Demogorgon's junk?
On the other hand, Wizkids goes very standardized, by the book (literally, as in official WotC or Pathfinder books for the most part) with their miniatures, which gives them a lot of consistency. Their small and medium miniatures are okay if a bit bendy, but many of their larger miniatures are fantastic, and their range is extraordinary.
Steamforged do top quality design and, similar to Reaper, aren't afraid to put their own stamp on the creatures they depict. Their "Encounters" miniatures are great, not to mention the many miniatures they do for board games and more niche RPGs.
Dungeons and Lasers (Archon Studios) are starting to compete with Games Workshop in terms of quality HIP miniatures but with a more traditional fantasy aesthetic. If you don't mind some assembly, they are probably the best currently available for D&D-style miniatures, though they don't have anything like the range of a Reaper or Wizkids. Yet. They also do some cool, quirky sets for other games and genres.
And even though they are going through tough times, I'm a huge fan of the Cthulhu mythos, and though basically every miniatures company has models that fall into that category, C'mon games really specializes in it, and they've got you covered if cosmic horror is your thing.
The Golden Age of Miniatures probably peaked around 2020, but we still live in a miniatures bonanza that my twelve year old self could only have dreamed of.