Most definatively Nintendo 64.
Great Multiplayer? check
Plenty of great games? check
reliable? check
Fast loading times? check and check

maybe gamecube because it has a better controller... but I would say the games aren't as top notch.

PSP Go with Custom Firmware. The PSP has some of my personal favorite games of all time and on top of that, it's just perfect for all that GB, GBA, NES, SNES, Master System and Genesis emulation goodness. On top of that, the PSP Go is one small sexy system with a really great battery and some cool non gaming features.

Best console ever made imo.

PS3

I would take a GameCube with a gameboy player attachment!

WiiU

Backwards compatible with Wii games, WiiWare and the Wii virtual console(NES, SNES, N64, SMD, SMS, NG etc...)

Getting all the new WiiU games, eshop games and virtual console games.

Resident Evil 4 is on the Wii and Wind Waker is on the WiiU, so those are covered.

I would miss Eternal Darkness, Paper Mario TTYD, Super Mario Sunshine, Warioworld etc...but the WiiU might get GC games, so I would bug Nintendo for those

PS1, PS2, PS3 games, doubles as a bluray player, media center, and a space heater.  Those cold winter nights aren't so cold while gaming on a 60GB PS3 :).  Saves money from not having to turn on the heater.

PS4

Playstation 2

It would be the NES because unlike modern games, it's software is infinitely repayable. I have gone back and beaten SMB over and over and over again and I originally played it for the first time in 1986 as a child. I can't say the same thing for MGS or GTA.

Original Wii.