Disgaea 2, and I love the first Disgaea.
Senran Kagura 2, took the established characters from the first game. Upped the ante, threw in some crazy bosses, and toned down the fanservice.
Senran Kagura Estival Versus, and on that note, the spin-off sequel is a lot better than the original too. Shinovi Versus is just a rehash of the first game with a barer plot and a different school and doesn't add much. Estival Versus is an original story that adds much needed character development to those two new schools.