This is a rather simple question, but you must remember some key things before answering it:
- Sharks are not fish. They're called elasmobranch fish and are closer to rays.
- Whales are also not fish. Whales are MAMMALS.
So, the biggest fish is:
The whale shark. It's name might suggest it's either a whale, a shark or a mix of the two, turns out, it's neither of those and it has only received that name due to its whale-like size and shark-like appearance which you can clearly see on the image below.
Hope it helped,
BioTeacher101