Unfortunately apparently the final game in the series - GK 3 delivers a nearly perfect adventure game. The story, dealing partly with similar topics as for example the Da Vinci Code only immensely better, is deep, rich and absolutely gripping. The characters feel real (and you know it's not as simple as it seems), and something's always happening, even if you don't happen to notice it yourself. In fact it's one of the most replayable games ever, even if the actual storyline doesn't change per se, as there is most likely so much you have missed the first time that it's likely you'll see everything in a completely new light on a second time (and that's a good thing).
The engine divides opinions, but I find it to be the greatest interface in an adventure game so far - one that immersifies you in the world like no other. You feel like you really are in the world - instead of observing it - and you can explore every little part of the locations.
Aside from a bit of possible irritation near the end, there is really only one - very infamous - puzzle that really stands out as perhaps "bad", but it is so greatly outweighed by everything else - including what is almost always said to be the greatest puzzle of all times (a collection of puzzles, one might also say) - that it doesn't matter one bit. Also, there is so much to do that isn't mandatory. In fact in a retrospect it is surprising how much ahead of its time GK3 was, and perhaps still is, even if there were major deadline and budget issues. And the slight stagnation of the genre.
After it's through, you can't but feel melancholy thinking about what is apparently not to come after - GK4 lost somewhere in the depths of a visionless business. But could there still be hope?