I'm too early in the game to give a proper answer, but given its sandbox and open ended nature, I can see myself playing it for months and even years. The only other game I've played continuously for such a long time is Elite Dangerous. In terms of playtime it's likely to become one of my most played games ever, and therefore one of my favourites. Then again given it's nature, it has no story or proper characters, which makes it harder to love than more emotionally involving games like Mass Effect, Xenogears, etc, so it s very difficult to classify it compared to those narrative games. Attachment comes in a different way for a game like Ksp, I think it comes with the time spent on it, which makes your involvment grow as you play. All that to say, I guess it will eventually be in my top few games, but I must wait a bit before I can really say It's definitely one of the more original games, and for it's type it's very well made and complete.