Well, when it comes to 'adult', it usually means to me as 'most dramatic'
And in this case, the most 'adult' is Colloseum.
Reasons for this: You play as an Ex-Snagem (Is that what there were called?) member, the pokemon were EVIL and it was up to you to turn them good, you had to do so at a shrine, heck, you start the game out saving a girl who got kidnapped and put into a bag!!
Then again ... D/P had some dramatic turns in there too, what with the team Galactic leader going all evil, then you actually having to take pity on him, then the fact you could face GOD ... and there are so much more ...
Darn, shoulda voted for D/P