I remember back when I started in this industry the 3 top ones were online gaming, pharmaceuticals, and porn.
These days I would still expect those 3 to be in the top 5 along with credit cards (which I have heard feedback from a few people in on it and the money is HUGE).
I would also expect something like baby wear would be massive too only because mums find it hard to get out to the stores. A friend of mine owns a baby store and has all his merchandise online and he reckons more people buy it from his website than from the store itself and the market is huge.. and thats a local store. Imagine what you’d do with an aff site specializing worldwide…
Just my 2 cents.