Metroid!
The fifth one, in release order, is Metroid Prime.
edit: Oh wait, that's not even quite right. Fusion and Prime were released the same day. So they are 4th and 5th. Haha... oops.
Final Fantasy maybe.
V is my favorite gameplay wise, but ask me on a different day and I might say VI or IX is the best
Mega Man on Game Boy.
A pretty good argument could be made for Etrian Odyssey V?... except that the fifth entry is actually EO Untold. Though that's pretty good too. Eh, it's a consistently good series. My fav is probably EO III.
I'm sure I'll think of more if I keep trying but I'm doing badly so let's stop there lol.