As far as Mega Man goes, I really only like Mega Man X. Castlevania, on the other hand, happens to be my all-time favorite video game series. Obviously, I've been playing a lot of Castlevania Anniversary Collection, though I generally prefer the Metroidvania approach, with Symphony of the Night being my all-time favorite video game. Symphony of the Night just continues to surprise me, and I've probably played through the game well over a hundred times. Also, Alucard's one of my favorite video game characters, and I love the personality surrounding him in Symphony of the Night (especially because of the attention to detail in how he interacts with the game world and others around him).
Castlevania's just such a weird series, and it's hard for me to not be completely enamored by it. I love revisiting the same areas from game to game, albeit with new surprises thrown in; the concept of the castle itself as a "creature of chaos" that can change form is just really cool to me. I love the permanence of Dracula as a threat, as it really keeps things open as far as the potential for new installments in the timeline goes.