Yeah, you are right, it is hard to define this kind of music, there are many subgenres. Megadeth is tougher because they have thrash metal albums, heavy metal, and rock albums. Anyway, I tend to classify distinct styles, rock, metal and extreme, for example and that is enough.
My favorite kind of music is metal that is more melodic, and extreme metal (death metal) , but I also love rock and softer music and listen to bands like Muse, a Perfect Circle, Pink Floyd, etc, though I know less songs. You list good bands. Though, I have not heard much songs by Creed and Alterbridge (I know Blackbird, which is an AWESOME song). Maybe I'll check them out.
My favorite bands though are Iron Maiden, Tool, Death and Blind Guardian. You may like Iron Maiden, they have 15 albums I think, and many of them are good.