I'd have to go with the Wii.
I momentarily considered E) Others, with the runners up being N64 & GC, but then realized that Wii can play GC games, and all the best N64 games are available on Virtual Console.
hmm... the above makes me sounds like a N fanboy, which isn't strictly the case (certainly have other systems), I think I just have derived the most fun from N systems in the past...