I don't know if anyone else here is a Valis fan, you most likely never heard of it, and if you have and are thinking "aren't those hentai games?", then you're thinking of the rebooted text adventure which reuined the series. The originals were not hentai games. It's worth checking out for CV fans, as it's probably the only retro series with character development and deep story outside of any text adventures or RPGs, and they're side scrolling action games with some CV inspired bits. Plus, THESE GAMES HAVE AWESOME MUSIC! This'll be a long one, I''l quickly run through the main games in the series:
Valis 1: Basic side scrolling game, Ninja Gaiden style cutscenes, but with a more interesting story. Takes a note from Megaman with a new power up after every boss, though you can't select stages. You can find it on PC-98, MSX, Genesis/MD, PC-Engine CD, with the Genesis and PCE versions being the best to get (stay away from the dreadful MSX version).
Valis 2: Very dark story, perfectly executed with brilliant music and graphics. Gameplay can be a little finicky on my X68000 emulator, and some story scenes can be uncomfortable, but for the time, it has a very dark story with one of the most evil game villains of all time. It adds new schmup style levels, with Yuko floating, which adds some variety with the platforming stages. There is an english patch of the X68000 version, and I highly recommend the game. Also on PC-98 (more stable version of the X68000 game) PC-Engine CD (sucks, bad graphics, mediocre gameplay, overly long stages, a barebones retelling of the computer version's plot, but if you prefer a more lighthearted romp akin to Valis 1, 3 and 4, the PCE verison is for you).
Valis 3: Things get VERY CV here. Finding characters to join your party, like CV3, and one of them has a whip as their attack. Takes a step above CV# though, where you don't have to leave one character behind. Once a character joins your party, they're there to stay. It's a fantastic game on the Genesis/MD, but the PC-Engine CD version has VERY slippery controls, though it's usually the preferred verison among fans (I'd go for the Genesis version personally, tighter controls and easier difficulty).
Valis 4: Exclusively for the PC-Engine CD in Japan, like Valis 1's PCE release. IT has a good story if you read a translated fan article, and very good gameplay. The graphics are also fantastic, and it's a step up from the PCE version of Valis 3.
Super Valis 4: Name sound familiar at all? Most people are familiar with this SNES exclusive to the series. Even though it sounds like SCV4, it's actually more like Dracula X to Rondo of Blood. We all know how Dracula X is considered a barebones remake of Rondo, and SV4 is a barebones remake of Valis 4's PCE version. Did they feel like ripping off CV that much? It's a pretty good game on it's own, but instead of projectiles, you use the Valis sword like a traditional sword the whole way through, using a neat little powerup selection system to use sub-weapon type attacks, armour, or health refills. No extra characters, and only one animated story cutscene at the end, which is disappointing, but still a good game on it's own right.
Syd of Valis: I'd only call the MSX verison of Valis 1 worse... OMFG, this game is bad. Sure you can have fun with it, but it's NOT recommended. It's a Super Deformed version of Valis 2 for the Genesis/MD. You have to memorize patterns, and you use different suits you gain from beating levels to beat parts of later levels, which is fun, but it's incredibely unpolished. In fact, the credits in the english versions aren't even translated... so you get Japanese credits on an American or European cart.... wow. So disappointing the X68000/PC-98 versions of 2 didn't get a serious Genesis/MD port.
Ignore the Valis X games at all cost! They were a cash in, and just hentai text adventures, no substance whatsoever. They ruined a classic series and now everyone just thinks all the Valis games are hentai. If you're a retro fan, Valis is a great undiscovered series, and is really easy to get into with it's simplicity in gameplay. It's major selling point though is how it did many new things in the world of story telling, even if the gameplay was never very original.