It's definitely the PSX version.
I'd say the PSP version, since I'm a huge fan of the new voice acting, but there are a lot of programming errors, many of which are game-breaking. Also, the text isn't aligned with the new voices. It replaces the "I Am the Wind" credits theme with "Serenade of Sympathy".
The Saturn version? Yeah, it's bad. I still appreciate it for Konami at the very least attempting to make new things, but the two new areas are entirely recycled, save for the new enemies and music tracks. It has the Skeleton Leader boss, as well as a Maria boss fight. Also, the Godspeed Boots, which allow Alucard to sprint like Richter. Unfortunately, the game is as bad as people above me have stated. Some areas were split into two, or taken entirely out of the game, also. Alucard doesn't have a map button either (instead, the questionable third hand was introduced).
The Xbox 360/PlayStation 3 version is pretty cool, since it's pretty much the PSX version. The enhanced graphics are kind of ugly (they just blurred the sprites), but it can be switched to the original. The slowdown present in the other versions (like after defeating a boss) is not in any way present in these versions. Unfortunately, it also suffers from programming errors, and the three CGI scenes of the castle were removed completely. It replaces the "I Am the Wind" credits theme with "Admiration Towards the Clan" from Lament of Innocence.
Esco's hacked version... Well, he said it himself and it's pretty much common sense. It won't be done anytime soon. I can't exactly say whether or not it's better than the other versions yet, since it's only at the Prologue stage.
All in all, yeah, go for the PSX version first. Then move on to the others.