Any class and subclass listed in the Player’s Handbook, in Tasha’s Cauldron of Everything, or in Xanathar’s Guide to Everything is allowed, with two exceptions (see below). That’s a lot of options, so I’d recommend trying to identify a class you’re interested in first—I’ve included a document with a brief summary of the main features for each class in the zip file. You can also look at this summary as well.
After picking your class, look through the documents for that class in the zip file; the summary document has a list of all the available subclasses, and there are PDFs from the main sourcebooks with the details, snipping out just the relevant sections.
There are two subclasses that are not allowed: Clerics with the Peace Domain, or Twilight Domain, both from Tasha’s Cauldron. Basically, they have some features that tend to make encounters very difficult to balance.
If you find the options for subclasses overwhelming, I would recommend just looking at the options in the Player’s Handbook and ignoring the other books. The point is not to find the absolute best option, it’s to find an option you think would be fun and interesting to play!