You might want to have a look at this thread I started on
multi-heroes. Not definitive, but it's got some additional information.
I don't think there's any comprehensive list of vanilla game heroes specifically, but the units themselves are listed in Edi's DB and in lch's wiki. You can make some assumptions by perusing the AllUnits tab; if the unit name is Hero, assume it is. Some are documented in the Notes of that tab. Some you can infer by the fact that they have a unique name.
If you're using CBM, many more heroes are added by the "Worthy Heroes" portion of that, so you can see them documented in the mod text.
If you're up to the task of doing the documentation, it would be a nice resource :-)