Atari 5200 because it was my first system and I have a nearly complete collection, although I'm certain I've put more hours into the few games I had for the NES (the Atari catalog leaned heavily toward arcade-style games that were originally designed to eat quarters).
By number of games owned, the Wii/Wii U win easily, but I never really felt that sentimental about either system. I know the Wii U was widely panned, but I thought it was a nice improvement to the Wii in almost every way and had some great games that were basically copy/pasted onto the Switch. I just wish they'd done more with the asymmetric gameplay.
For modern(ish) systems, Switch is my favorite, as the PS4 just doesn't have that many games that appeal to me.