As far as I am aware, we don't know. He has claimed to have 7 Ph.D's, and we have no reason to doubt him, but he's never explicitly rattled off what they were. Given how many he has, we can probably assume he has degrees in all the fields of science where he's an acknowledged expert. These would include:
- Radiophysics: Fury claims he recruited Banner for his knowledge of gamma radiation
- Biochemistry (or bioorganic chemistry, or organic chemistry, or something similar -- maybe all 3): Stark needed Banner's help to merge JARVIS into an organic host.
- Nuclear Physics: In Avengers, Stark commented on Banner's research work in "anti-electron collisions"
The first two are, I think, the classes he taught at Culver, which makes those two almost a lock.
Also, while it probably wouldn't be included in his list of Ph.Ds, while in India he demonstrated pretty significant medical knowledge, so he may have an M.D. as well.
What the remaining 3 or 4 degrees are is unknown, but maybe we'll see him use more of them in future Avengers movies.