Having gotten Aquaria from a Humble Bundle a while back (it was the last Humble-Bundle-with-Android) I started playing it a couple weeks ago. It is a beautiful game, and it is hard. Maybe it's hard for me because I'm not really a hardcore gamer and my ability to figure out boss-fighting strategies is basically nil - or maybe it's hard for everyone, I don't know. But despite the many moments of lip-chewing tension, usually followed by another death, I really want to keep playing. It has a very strong exploration component and the desire to see what's in the next cave, or just around the next bend, keeps me in for hours at a time.
All to say, this game should really, really be here on GOG.