When it comes to printer filaments you generally get what you pay for. But when printing functional parts you also have to know what is geared for your needs.
PLA is the most common printing filament available and the one everyone generally starts with. It is cheap, easy to work with, rigid, and weak. Functional parts generally shouldn't be made in PLA but a few makers have proudly created designs that are PLA compatible.
In the early days you had two choices: PLA or ABS. PLA was what you used most of the time but if you wanted something functional ABS was what you had to use. These days ABS only has an edge in a handful of edge cases and a user capable of printing in ABS is usually able to print more advanced materials as well.
PLA+ is a hybrid material that has gained popularity with the Ender 3 crowd for being fairly strong and rigid while remaining roughly as easy to work with as regular PLA. It's a popular choice for light use pistol frames and it seems a lot of recent scene releases aim to be PLA+ compatible. If you want to print functional parts on a cheap printer without dedicating your spare time to mastering 3d printing this is the material for you. On the other hand it will deform it hot temperatures and there's no real standard for it.
Petg straddles the line between low-end and mid-range as the best filament you can print, with some difficulty, on a stock Ender 3. petg is an all rounder; strong and sturdy with just a little bit of flex to it, it has a reputation for being inadequate for pistol frames and generally being a runner up choice for all major projects (no real pros to capitalize on or weaknesses to design around).
Nylon is much more demanding than simpler filaments and is often out of reach for low end printers or inexperienced operators. It can be a bit flexible, but it is an excellent all rounder; being able to reliably work nylon is a worthy end goal for new Fosscaders. Its biggest limitation is the price point; while nylon filaments are becoming cheaper they are still expensive enough that other 'good enough' filaments remain more popular.
The odd one out, flexible filaments can be very useful in certain situations (grips are a popular choice) but no one would seriously suggest printing a receiver in something that you can bend around with your bare hands.
If raw petg is too flexible, carbon fiber petg is just right. Easier to print than its higher temp composites, carbon fiber petg is a fine choice for many projects provided you like them in black. The main reason not to use this is if you can print this you can probably print carbon fiber nylon too.
High end filaments are typically blends of mid-range filaments that shore up their weaknesses. High end filaments aren't that much harder to print than their un-blended counterparts; they're just really expensive. And this is one area where price really does equal quality. Good filaments have material property sheets; bad filaments just look the part.
Polycarbonate boasts high strength and temperature resilience. It also requires a high printing temperature and it tends to warp heavily. Printer-wise the requirements for printing this puts it just on the edge of most mid-range printers. The general opinion is that it's not worth it; though good for small parts operating in continuously hot environments polycarbonate's weaknesses make it poorly suited for most fosscad applications.
If you want a solid heat resistant frame look no further. Glass filled nylon differs from carbon filled in that it's a little bit more impact resistant and comes in more colors (Nylon can be dyed fairly easily). Carbon fiber on the other hand is a little more rigid and comes in exactly one color--black. Both are good, but expensive, choices.
todo my limited experience is this is actually easier to print than regular PC as it seems to warp less. No idea how it compares to GF/CF nylon.
All of the above temperatures have a minimum print temperature of 300°C. That's because 300 is about as hot as a desktop printer will go. Beyond it lies a magical realm filled with aerospace engineers and filament rolls that cost hundreds of dollars per kilogram; this is the land of PEEK and it is a land few consumers have traveled to.