Pokemon for known reasons, once you get over your own barrier that is telling yourself it is a "kid game" it can be quite funny. if you get into it, especially emerald, you easily can have 150 hours or longer fun with a single savegame.
Final Fantasy Tactics Advance; fine game, turn-based strategy roleplaying. Unlike the other FFs, you have to also move the characters on the battlefield. Bad story and cast IMHO, but the gameplay and the customization makes up for that. You have a total of 300 missions which can easily take up 150 hours to complete.
Fire Emblem & FE: The Sacred Stones: At first glance a bit similiar to FFTA, but it offers less customization but a very much better storyline and cast as well as dialouges. Oh, and it definitivly harder and more tactical than FFTA.
Castlevania: Aria of Sorrow: A nice CV game, taking it roots to Simphony of the Night. If you liked that, you will surely like this as well.