I'm a huge fan of the KH series. One of the most complex stories in gaming history spread over 8 games with more to come. Suited for both children and adults. Clear experimenting in games which barely had a negative effect on the game other than "not my favourite". Bugs are rare, at least I never saw one. Speedruns and strategies are still being developed years after the release. Difficulty depends more on the players skill and knowledge than simple brutality of the AI or mechanics. It's one of those games people will even play in japanese just for the experience. Who cares about language barriers.