The one with most potential, for me, is The Adventure + Belmont's Revenge.
We got so much going on on those games. We have a father-son relationship, we have a passing ceremony, we have a nice plot involving Dracula manipulating Christopher's son. We could get two movies out of that EASILY, but even easier would be getting one VERY GOOD movie out of both stories;
The movie would start with introducing us to Christopher's life. His family and his infant son. Shortly after, Dracula is introduced (which could also tell us about Trevor's previous exploit against Dracula) and Christopher's mission is set. From that, fast action until Dracula is defeated (which would be halfway into the movie). Then we have the events of Belmont's Revenge. Christopher is old, and is at his son passing ceremony, Soleyu disappears with the Vampire Killer after wreaking havoc and promising to bring Dracula back using the power of the whip. The whip was already introduced at the movie's start as being the "ultimate weapon" against Dracula, and now it's gone. Christopher goes after his son, and finds him at Dracula's Castle's ruins (for us, of course, it'd be no secret that this is Dracula's doing, but it'd be nice to introduce Soleyu as the "villain" just to subvert it later reintroducing Dracula). They battle, Christopher using another whip against Soleyu's Vampire Killer, and Christopher wins, establishing that being powerful is more than simply wielding the whip. It's then revealed that Soleyu is possessed just as we see Dracula being "exorcised" from Soleyu's body. He takes shape, and attacks Christopher, almost killing him., because now he's powerful from having leeched Soleyu's blood/soul and Christopher is old. But then Soleyu steps up, and fight alongside his father to defeat Dracula once and for all.
Notice how I left out the "four castles" thing. This, I think, would be silly to translate into a movie. It'd be much better (I think) if Soleyu was simply introduced as a villain, but then revealed to be possessed.
A post-credit scene would start with the words "100 years later...".
We see people celebrating Easter as a this song plays:
https://www.youtube.com/watch?v=4tDYMayp6DkThen we get a badass contrast of jumpcuts. We see a red haired man marrying a young woman, colorful decoration and happy people, and at the same time a dark ritual, inside a tattered building, going on (all with the song above playing for both). The scene jumps between both events quickly between the priest saying his lines full of hope and promise, and the dark priest saying his chaotic, opposite lines. And it finishes with the newlyweds saying their votes just as we see Dracula opening his eyes inside his tomb, and getting the final bit of the song distorted. Yeah, a cinematic representation of Haunted Castle's intro.