It's really not easy for me to decide. I love NES and Megadrive so much for their 'simplicity', the visuals were beautiful in a special way you can't recreate in modern graphics because every pixel mattered, and the way the games worked, and the ways they were designed.
Even though I am part of the generation that came after that really, I always prefer to play those older games when I have some free time to waste.
But then, I really love how later generations added a lot more complex possibilities in engines and game dev. I mean when I got my first Playstation 2 and Gamecube as a kid they really blew my mind with how much the games could make me feel like I was really seeing into a whole pretend world.
Also, I really love more types of user interface and input devices (maybe those are not the correct terms) like Wii remotes. Although I know many people did not like it, the Motion Plus Wii remote is still my all time best controller setup for anything ever, it really is relaxing and fun to me to use it. Like I can spend hours playing Rune Factory with it, or just mess around on an FPS for a minute, or use it to play DOS point and click games from all around my apartment without any problems, it's really great for me. I want to find a way to control my computers like that lol.