As far as "Ico"s go, play the very first "Ecco the Dolphin" game. It's an absolutely
amazing and one-of-its-kind artfag game, that you'll remember for a long, long time. It's not really any kind of openworld (there is a very clear level-based and key-based progression), it's kind of an arcade game in more general sense (not a platformer, since it only tries platforming for a handful of times in the entire game) and it has kind of a crappy hit detection, but all the art things compensate for that tenfold - and there is still, up to this day, absolutely
nothing quite like it.
Why should you play it and what's so special about the art thing? Let's put it this way: the game starts as a simplistic immersive (minimal hud, reliance on actual dolphin abilities - sonar pulses and echolocation, hunting for fish in order to restore the strength, battling currents, restoring extremely limited air supply - in the game mechanics, etc.) "dolphin simulation" with a couple of fantastical elements here and there - and evolves, one step at a time, into something progressively more and more warped and weird. There is an absolutely unique and coherent ingame setting with an absolutely unique atmosphere (the setting itself is somewhat thin, but everything is there for a reason); a coherent, interesting, really well structured and well told story/narrative (don't forget about "dolphin simulation" part) - and yes, I'm aware that I'm claiming that there is a well written story in a game about a bloody
dolphin; an absolutely amazing soundtrack (CDA one) - and, what's more important, the game has the balls to direct all the aforementioned things towards One Single Moment (not really a "moment" - you'll see what I'm talking about when you get there, it's impossible to miss) which you
WILL remember for a long, long time after completing the game (and which is incidentally one of the greatest trolling attempts I've peresonally ever seen in videogaming). Interested enough already?
There are three major versions: Sega Genesis/MegaDrive version (soundtrack completely different to the other two, I'm not a fan of it and I absolutely don't recommend playing this version first) which is also incidentally a Steam version, Sega MegaCD version (the same 240p graphics as Genesis version, full CDA soundtrack; the most "canonical" version of the game as far as I'm concerned) and Win95 CD version (480p graphics with many sprites completely redone using the assets from the second Ecco game; more responsive controls; considerably abridged version of MegaCD version soundtrack due to the CD constraints and all those "prerendered cinematic cutscenes" + a couple of little and questionable changes to the game's most memorable level; a handful of prerendered cutscenes lifted from the second game, which you should totally skip any time you happen to activate one, because they absolutely don't fit the game's style, spoil the things before they actually happen and frequently make the game hang or terminate itself).
Oh, and here is some of that aforementioned
music.
It's a pity that the sequel (Tides of Time) was so much weaker (despite all the increased gameplay variety). Never played Defender of the Future, but it seems to have its share of negative impressions around the Web.