It would have to be PSP, followed closely by Dreamcast.
PSP was so far beyond GBA and DS it was unbelievable, the huge screen, console graphics, analog sticks, optical media, it was crazy tech.
Dreamcast was really insane if you think that N64 was released just 2-3 years before. Going from that low poly blurry fog mess to crystal clarity and fluidity of Dreamcast, complete with the triggers controller was spectacular. Even compared to PC, it was on its own league.
For the same reason PS2, Xbox or GCN didn't feel as advanced, they were just slightly upgraded from Dreamcast. And to me, both Xbox 360 and PS3 felt pretty ordinary when they came out, we had seen similar graphics and fluidity on PC already.
N64 did feel pretty advanced st forst sight with the anti pixellation measures and forced AA, but when the fog and blur took over those empty game worlds it felt almost instantly dated.