I got into these way before most people did, apparently, so I'm a fan of several that have been lost to time.
First and foremost: RPGMP3.
As far as I'm aware, they were the first. They did a recording of World's Largest Dungeon in 2005.
While the audio quality on WLD is dire, they eventually moved on to binaural surround recording, which makes their recordings extremely easy to follow if you're wearing headphones. Eventually, when their in-person groups fell apart, they moved back to stereo, but what're you gonna do?
Their website hosts so many different groups and games that I can't vouch for most of them at this point, so I'll have to make some specific recommendations.
The "RPGMP3" group itself does mostly bombastic, combat-heavy D&D/Pathfinder stuff.
If I was going to recommend one thing, it'd actually be their Rolemaster campaign, just for how weird the whole thing is. It's not finished, nor will it ever be, but it was amazing.
The "Bradford Adventuring Guild" are rather more subdued, but still tend towards combat stuff. One of their main members was also a main member of the early RPGMP3 stuff (before they left England), and is also a main member of one of the groups I'll be talking about later.
Their Warlords of the Accordlands game spanned several years and is 105 episodes long, an episode typically being over 2 hours. It started out in stereo, but they got a binaural setup a few episodes in and then it becomes much easier to listen to. I've listened to this whole damn thing twice. There's so many hilarious moments.
"Whartson Hall" mostly play smaller, storytelling-focused games, and their games tend to only be a few episodes long. Like the main RPGMP3 group, they initially recorded everything in person but eventually the group fell apart and now they do remote stuff. I thiiiink they've moved to another website but I forget what it is currently and I'm mostly familiar with their old stuff anyway.
Their one-off of Point Blank is one of the most hilarious things I've ever listened to, and I listen to it at least once a year at this point.
More seriously, I absolutely love their Sorcerer Cyberpunk game.
A lot of these might be hard to just jump into, but the early recordings have truly terrible audio quality. Those old ones are where I started, though, and I know it's easier to get into one of these if you already know someone doing it, so it was helpful for me how they all kind of branched out slowly
Now, on to a different website: Yog-Sothoth.
They were sort of a spinoff of RPGMP3 (two members of Yog-Sothoth were in every early RPGMP3 recording), a single group who play nothing but Call of Cthulhu and related games.
The vast majority of their stuff isn't available for free, but perhaps their best game of all time is: Horror on the Orient Express. You do still have to make an account to download, though.
All of their stuff is very high quality, but since most of it isn't freely available, it's hard to recommend much