Not so many to choose from today but in general Oozes are a lot of fun to use. In most of my campaigns they make a few appearances. They can be in a random hallway, drop from the ceiling or maybe inside a pit trap. All great and deadly.
Sometimes the classics are the best. In the case of oozes that is true. My favorite is the gelatinous cube. I think they are just an awesome invention to mess with dungeoneers and to make traps and rooms interesting.
The fun thin about these cubes is that you don’t see them coming. You can walk through a hallway and suddenly you are stuck in a colorless pudding who tries to eat you.
The best way to use them in later levels is in addition to traps. Or a creature that uses these cubes to guard things.
In a one-shot I once encountered a talking gelatinous cube and that was also a lot of fun!