I really like the GBA SP. The backlit screen was awesome at the time. :D Plus it was comfy and you could pretty much play with one hand, unlike the GBA. N64 was great because of the multiplayer aspects and I have lots of nice memories of that. I also really like the DS Lite, the rounded corners and compact size are nice and there were a lot of good games for it.

It's really hard to pick just one. xD But I probably love the Nintendo64 most, following by GBA SP then DS Lite.