Ico is lesser known/underrated because it's the spitirual "prequel" to shadow of the colossus. The game first released on PS2, but you can play it on PS3 from The Ico & Shadow of the Colossus Collection which is a HD version of both games on a single disc.
I played the PS3 version on RPSC3 and it worked flawlessly except for a few crashes.
The game is an experience unlike any other. Sometimes we say about some games "they don't hold your hand", like the Souls game. Well, this game will never give you a single hint, it's that mantra pushed to the extreme. It's best described as Portal without the portals. It makes the experience very immersive since there are no HUD elements, no voice telling you to do anything. It's essentially a puzzle game, but it won't feel like a puzzle game, it will feel like an adventure you are solely responsible for and for which the end result will be 100% your achievement; It's not a multiple endings game, but it's such a well designed game that the player will feel proud of the journey.
I can't say much more without spoiling, all I can say is please give this game a shot, it will change your perspective on games as a whole.
My rating, 10/10. There are 0 flaws with this game, not a single con I can nitpick about. There's barely any dialogue but it can easily make you quite emotional.
I played this on the PS2 and it's s fantastic experience.
Interestingly, the PAL version (and probably the Japanese version, too) has content that wasn't in the NA version. There's an extra puzzle, a semi-hidden alternative "funny/happy" coda after the main ending if you play through a second time, and some extra in-game options that are unlocked after you finish the game for the first time, including understandable subtitles for ALL characters, even ones that are normally speaking an unknown language. I'm not sure if the hidden weapon you can get in the middle of the game becomes a light saber on the second playthrough in the NA version as it does in the PAL version, but it may.
This was before video streaming sites, so there were many arguments on forums about how these things are in the game, no they aren't you trolls, yes they are here's a picture, that's obviously fake... and so on. It was interesting that once people figured out that the NA and PAL versions were different, there was a vocal core of NA players still insisting that it was all fake for quite a long time afterward.
I'd like to add an opposite take - not to disregard your opinion but to maybe tamper the expectations a little.
ICO is a game I really wanted to play for years. Besides being widely praised and held as a classic, it just looked interesting even though I didn't knew much about it (on purpose). All I knew was that a boy and a girl travel through some ruins and there's a button to hold hands - that's it.
After finishing it, all that comes to mind is: it was... fine?
I really like what the devs did (or at times tried to do). I liked the atmosphere, (limited) story and the general idea behind it all. Unfortunately, the good didn't manage to overcome the bad/mediocre parts for me.
Gameplay was mostly fine if not for the tedious combat which happens a lot. It was bad enough there were multiple times when I entered a room, saw enemies spawning in and thought to myself "I really don't want to deal with this right now" followed by a switch to Tetris DX.
I made it through, but the combat in this game is absolutely not good enough to carry this amount of it.
It's not even a case of it being janky (I can deal with that), it just wasn't engaging in general (even though, again, I do understand what they were going for).
Levels were interesting in concept but I think they went a bit too far in trying to make them look like a "real place" without filling them with enough details - the final effect is large, empty maps that could be vastly improved, even on original hardware.
Light content spoiler
I also expected some gameplay outside of ruins. Not necessarily a second part of the game but at least something. I'm not saying not getting my wish was good or bad, it's just something I thought would happen but didn't really affect my thoughts on the game.
Music was kind of there. It's not bad but for some reason it was completely forgettable for me. The only track I could remember after finishing the game was the save point song (listening to it actually makes me smile, not sure if it's just due to the song itself or how I associate it with not having to fight anymore).
I ended up listening to the OST on youtube and was constantly thinking "oh yeah, I did hear this song!" and definitely like what I heard, it's just that almost none of them really registered when I played.
It's been a few months since I finished the game so I think I mellowed out a bit on how I feel about it. Just so we're clear, I never outright disliked it. I'm glad I got to experience ICO and I did like what I played overall but I don't think I'll touch it ever again.