Pokemon and the Mario & Luigi series are my favorite.
For Pokemon, it's just the fact that even if the cities are the same, it's almost always a different game every time, plus, the challenges like nuzlockes and the competitive sides are extremely fun as well, even if I suck at them.
Mario & Luigi, on the other hand, always keeps you on your toes with it's action command mechanic. I honestly love the game for this specific reason. I don't like having to wait for a bar to fill up so I can attack (no offence to final fantasy fans, I love the series I just don't like the ATP bar tbh), I wanna be able to always be engaged, even in the enimies' attacks!