We have here a good graph that shows Shimano and SRAM rear derailleurs depending on the performance level.
This could be helpful if you want to buy a new bicycle and you want to have an idea regarding the overall quality of the rear derailleur.
Pretty much all very cheap bicycles that come with a Shimano rear derailleur have a Shimano SIS derailleur, also known as “flat face”.
This derailleur is far from perfect, has issues shifting but it’s a huge step from the no name derailleurs.
In 2018-2019, pretty much all entry level bicycles come with at least Shimano Tourney. These are reliable derailleurs.
The next step are Shimano Acera or SRAM X-3, both great derailleurs for basic mountain bikes with a price range of 200-350$ or even more.
Shimano Altus / Alivio and SRAM X-4 are the next step, we find them on bicycles in the range of 300-400/450$.
Leaving the entry level, we find the basic Shimano Deore, SRAM X-5. Great derailleurs, should work flawless.
Moving on, middle – high end, we find the rest of the derailleurs that come mostly on more expensive bicycles with a price of at least 600-700$ to 2.000$ – 3.000$.
Shimano XT, SLX+, SRAM X-7, X-9+.