Way back in the day we used Heroquest minis, but those have been lost to time. I generally prefer to use theater of the mind when playing IRL, but occasionally use coins or dice to give a general layout for more complex fights. Since I now play on Roll20, you can make your own custom tokens instead, which is really nice.
I buy just D&D fantasy. Our group has a lot of the 3e pre-painted figures and I will buy some random packs from the Wizards line or Pathfinder lines at the FLGS. We have some Heroquest with the larger spinning base taken off and some random animals or beasts from non-D&Dish games or lines thrown in. My brother has several of the Bones Kickstarter sets with hundreds of unpainted figures and he also likes to collect the paper minis that he can tote in baseball card sleeves. Between all of that, I do not tend to buy for myself anymore, but will pick up some randoms from the convention store and some for my son's collection- which he brings to the game for me to use. I guess I win either way.
Mostly prepainted minis from the old WotC and the new Wizkids line, mixes with some of my old metal miniatures from the 80s. I had some of the early Dwarven Forge sets before they became ridiculously expensive, and a few other complementing terrain items. I also have a lot of Battlemats from my D&D Minis days in the early 2000s. For non-fantasy, I use minis from board games like Cthulhu: Death May Die or Deep Madness.
For my one live game, one of the other players is buying all of us Heroforge minis. He seems to like building and painting, them. Before he got into that we used the best of what we had lying around; the dm had a decent collection built over the decades.
For my other live game (that actually hasn't gotten back to live yet), we use random pawns from board games, dice, coins, and plastic soldiers and zombies. Also some random toys for bigger monsters.
This is a two part answer.
1. I buy a ton of minis because I want to use them in the game but it rarely happens when I have that opportunity.
2. I buy a ton of minis with no intention that they'll ever be used in a game, but because I want to paint them but never get around to them all.
Basically, I buy a ton of minis. It's a sickness, really.
For day to day miniatures I prefer the original D&d minis. They’re hard wearing and there are some great minis in those sets. Pathfinder minis are a close second the sculpts are generally really good even if they don’t map exactly to D&d monsters. They can be brittle though. The D&D Whizz Kids models are generally pretty crappy. Bad sculpts, brittle and the scales whilst more accurate make the minis look quite underwhelmjng in cases. Pathfinder minis suffer the same but less so.
For PCs I think reaper are excellent. There’s something satisfying about metal minis for PCs and their catalogue is so huge you can usually find pretty much anything you are looking for.
For epic monsters you can’t beat games workshop. Their hydra, chimera, black dragon, cockatrice, jaberslythe, giants etc etc are simple beautiful. Though expensive they’re true display pieces when painted.
Speaking of epic, the Gale Force 9 miniatures are also beautiful, particularly the demon lords sets. I’ve shared this image a few times and particularly love Demogorgon and Orcus. That mini alone makes me want to play Rappan Athuk again!
Last but not least, I think the gold standard for scenery has to go to Tabletop world. I’m lucky enough to have a few pieces by them and they are a pleasure to paint.
All pictures are mine. Apologies for some dodgy paint jobs