I'll answer by saying which machine is my favorite from each of the decades you listed:
70sI think the Intellivision was by far the most far out system to be released in that era. I know Magnavox and Atari came first but the Intellivision was so far ahead of those two.
Plus, the Intellivision had Thin Ice with Duncan Penguin, by far the most bad ass mascot of the era because well... he
was the only mascot of that era!
All that aside though, the Intellivision was my Dad's favorite system as a kid. We still play World Championship Baseball to this day, and my Dad is crazy good at Space Armada too.
80sThis is actually kind of a tough one for me honestly, my brain wants me to say NES but my heart wants me to say Sega Genesis. Ergh, I'd have to go with the NES.
There are so many fun games I've played on the NES; Super Mario Bros., Mike Tyson's Punchout!!, Blaster Master, Mega Man, Kid Icarus, Metroid, Kirby's Adventure, R.B.I. Baseball, Tecmo Super Bowl, StarTropics, Tetris... Damn, just too many to count. Once again, this was a system my Dad turned me on to as a youngster. He had two NES systems and a collection of games which my Mom donated in the mid nineties unfortunately. In 2005 or so, he got bit by the gaming bug and re-bought the NES and Intellivision off eBay for insanely cheap and got all of his old games back too (I'm glad he did this back then as opposed to now because the prices would be like 10 times what he paid)
I enjoyed the NES a lot even though I had been exposed to the N64, GameCube and Game Boy systems by then and still have a soft spot for the system to this day. Even though I didn't get to experience this system firsthand (I'm 23) I am glad I got to play the NES.
90sHands down has to be the Super Nintendo
By the time I played the SNES, we already had an N64, but I'd be hard pressed to say I liked the N64 even one tenth as much as the SNES. I still remember a trip to my Grandma's when I was maybe 7 years old, all the kids would go to the basement to play video games and that was where I first experienced the SNES, playing games like Super Mario World, Rock 'n' Roll Racing and my all time favorite video game of all time: Donkey Kong Country.
This should be a fun thread. Can't wait to see what others post.