I hardly ever refer to games as "perfect", but for me System Shock 2 is one of the few games that comes close to that. The graphics are a bit outdated by today's standard, but the atmosphere has not changed at all. SS2 is probably the only game that has genuinely scared me. I remember the first time I played it, in a dark room at night, and how my heart rhythm probably qualified as cardiac arrest at several points. You don't need to have played the original System Shock to enjoy this game, but if you have then you'll be pleased to know that Shodan is back. If you don't know who Shodan is, imagine a darker and more angry GLaDOS.
This game has claimed many many hours of my life. The huge amount of tools available to you are nothing short of amazing, you can almost change every little part of your park. If you've ever like a Tycoon-game then you'll likely love this game as well. However... seriously GOG, $20 for this? Your reputation just took a small dive, this is the first time I've seen a complete miss on the price of a game. Would be 5 stars if not for this.