I played Street Fighter II a lot when I was young; alone, with friends and with my sisters, so 2D Capcom style fight games got a place in my tender memories.
On the other hand, I like superheroe comic books, so Mugen is a perfect fusion of both things. It's like a Marvel.vs.Capcom videogame but greater, and constantly updated with new chars and stages (and some creations are really well done), and where I am able to control the arcade fights and edit whatever I want (whatever I can, actually).
I'm sure Mugen, as game and as concept, is one of the best entertainment ideas of my age. Furthermore, I think it might teach some people that hard work could be fun, and how much gratifying is finishing a task.