Okay nice, I love this part.
Slipseer is a great resource, but I recommend also checking out Quaddicted for browsing. It's an older site, but it's the closest thing there is to a complete archive of every map/mod ever made. A lot of my recommendations will come from a position of someone who only really got into the scene a few years ago, but there's great stuff going all the way back to 1996 (well, okay, maybe 97–98).
I couldn't possibly recommend every map I like because there are way too many (and honestly, my memory isn't good enough). I'm going to stick to a few collections that I think work well as entry points.

Underdark Overbright is a good place to start, I think. It was made to demonstrate the Copper mod (a popular 'vanilla+' mod that polishes a few of the game's rough edges) and it's a nice high-quality episode in its own right. It'll also get you to install Copper, which is an important thing to have lying around if you want to play custom maps.

The func_mapjams were a joy to dig around in when I was just starting out. Each one has a specific theme, and most of them have a selection of really polished, high-quality maps from various community members. You'll hopefully get a feel for how different level designers approach their craft, and be able to sample plenty of maps without the pressure to finish any of them. I think jams in general are a great way to sample what the community is capable of—and unless the focus is specifically on mapping around a really tough limitation (like speedmapping), they're almost always a blast to play.

Dwell is one of the more popular mods of the last few years, and not without good reason. Currently only two of the episodes are finished, but they're both very polished experiences—the former is themed on Ancient Egypt, while the latter deals more explicitly with Lovecraftian notions of the sunken city of R'lyeh. Might be a bit tough if you're still new-ish to the game, but there's no shame in turning the difficulty down.

Most of these recommendations are episodes or map-packs (just trying to provide as much material as possible) but I really do insist that everyone plays Honey at least once. It was hugely influential on the last 10+ years of maps, to the point where people will say a map is 'Honey-style'. The use of scale and fog is wonderful, and it's a fun time to boot.
Lastly, if you like... you could always play my maps 🥺👉👈