Would be a tough decision but since I'd have to keep just one for all time, I'd have to say the Dreamcast. It has the best community of gamers surrounding it, it has great games, it will be playable online forever (theoretically) and the games don't require patch servers to function.
I have a group of family members and friends that plays PS4 together once a week, it would be hard to leave them. I'd also never get to play Shenmue III which would be terrible. At the end of the day, the PS4 community will fade away and many of its games will not function at a basic level without patches accessed through PSN. Someday the PS4 will be a hollow shell of a console so, thinking longterm, I'd have to stick with our beloved Dreamcast.