I'll try, but my choices are probably inept in several departments
Since youre using this as a CD of 'genre entry points' (that sounds a little weird..) I'll go with things that I recall I identified with any given genre as early as possible and try and avoid anything thats the result of longer exposure or knowing more now/being more anal about what category stuff falls in.
Classical - Beethoven, Fur Elise.
Opera - More or less anything from Carmen
Heavy Metal - Metallica, One
Pop - Micheal Jackson, Bad
Reggae - I shot the sheriff, Bob marley
Hip-Hop - Pass. I quite like Saul Williams, Penny for a thought, but I'm given to believe thats a little off the beaten path in hip hop land and that I have no idea what I'm talking about, and 'like' isnt the question anyway.
Jazz - So What, miles davis
Latin - I'm dimly aware it exists, no more.