Castlevania Dracula X: Rondo of Blood
Released in 1993, Castlevania Dracula X: Rondo of Blood was hailed as one of the greatest of the Castlevania series. Containing a mix of branching paths, different playable characters, improved artwork and music, and a whole lot of replayability (gotta find every secret), Rondo of Blood had everything a Castlevania fan could hope for.
That is, if you lived in Japan. Released only across the pond, the original version of Rondo of Blood never had a "proper" release in the West -- probably because it came out for the TurboGraphix-16 CD-ROM (PC Engine in Japan), a system that never caught on in the States. Porting the game to the U.S. wouldn't have been economically feasible, so most American fans didn't even realize they were missing out on one of the great Castlevania games, at least not until the internet came along.
Rondo of Blood was, in just about every way, an improvement for the series. Hero Richter Belmont came to the party with the family's whip, the Vampire Killer, and a whole assortment of sub-weapons -- sub-weapons that he could release in over-powered attacks called "Item Crashes". During his adventure through Dracula's castle, Richter could find various other characters -- one of whom would join up as a second playable character, Maria Renard.
Maria, 12-year-old vampire hunter, was a huge innovation for the series. Previously, the only time another hero could join the adventure was in Castlevania III, and they certainly were supplemental to Trevor Belmont. Here, Maria featured abilities and powers than made the game easier (than with Richter), and Maria proved to be just as capable as the Belmont. Maria's main attacks were animals -- doves as her primary, and other spirit animals are her secondary sub-weapons and item crashes. While she wasn't as strong as Richter, her powers were at times better, and the fact that she could double-jump made navigating many of the dangers much easier.
The two heroes could find multiple paths through the castle (the main path and a second, secret path), and the computer would track all the passages found, giving the player a percentage of the game explored. This system encouraged players to explore every inch of the game, as much for "completeness" as to ensure they got the best ending.
On top of all the features and ways to play, Rondo of Blood had some of the best music to date. Using the full power of the PC Engine's CD-ROM, the soundtrack for Rondo of Blood was an orchestral/rock treat, and even now, over a decade later, is still considered one of the best soundtracks in the entire series. The graphics, though good, were not as drastic an improvement, though, but coupled with the stellar soundtrack, Rondo of Blood was considered to be quite the audio-visual treat.
In just about every way, Rondo of Blood was the game to get, which explains why, for the longest time, there was quite the resale market for original copies of the title (at one point going for upwards of $300 on auction sites). American audiences did recieve a remake/reimaging of Rondo of Blood, Castlevania: Dracula X for the SNES, but that game wasn't nearly as good as the original. It wasn't until 2007 that the West got a true version of the original game in the Dracula X Chronicles, which featured a new remixed, 2.5D version of the game as well as the original (plus an enhanced version of the sequel, Symphony of the Night), finally sating fan desires.
Ports and Rereleases
As noted above, Rondo spent some time languishing, unable to be played outside Japan as Konami refused to port the title. Eventually, though, they relented, and the game was included as an unlockable bonus in the Dracula X Chronicles, although you had to play through the 2.5D remix of the original title to get it. If that doesn't interest you, there's also the PS4 port that came later, as part of Castlevania: Requiem. That version is the PSP unlockable mode -- the classic game with the proper, English translation -- so if all you want to do is play the original title (and not the PSP remake), this is the way for you to play.