Best decade was the 80's. Gaming was growing by leaps and bounds, and devs were not afraid to be experimental. Game developers seem to have gradually forgotten that novelty makes something more entertaining. In the 80's new and amazing stuff was coming out every year that had nothing like it come before:
1980 Pac-Man, the first maze game and arguably the most popular game of all time.
1981 Donkey Kong, the first successful platformer.
1982 Lots of popular and unique games, some of which defy classificiation: Dig Dug, Joust, Moon Patrol, Q-Bert, Robotron 2084
1983 Dragon's Lair, a game animated by Don Bluth. It was so beautiful that it still looks good by today's standards.
1984 Punch Out, the first big boxing (or fighting) game and also Duck Hunt which is as important as Mario for reviving consoles
1985 Super Mario Bros, 'nuff said. Arcade still going strong with Gauntlet.
1986 The Legend of Zelda, Castlevania, and Metroid premier at home with arcade still getting solid games like Rampage, Out Run and Arkanoid.
1987 Home console gaming starts to birth some of the greatest franchises ever: Mega Man, Final Fantasy, Contra, and Metal Gear. Double Dragon tears up the arcade.
1988 So many awesome home consoles games: Super Mario 2 & 3, Mega Man 2, Blaster Master, Ninja Gaiden. The Genesis launches in Japan.
1989 Too many home console games to count. The NES is an unstoppable monster while the Genesis is ramping up and the arcades are still going strong with games like Golden Axe. This is quite possibly the best time for gaming (especially 2D action games).
The 80's was so awesome, because so much happened. In spite of the fact that the home console market crashed, there were so many good games that it makes the 80's the best decade for gaming by far. In the early 80's the best games were in the arcades and not the home anyway. And those early 80's arcade games were revolutionizing gaming every year. In contrast the arcades had only one revolution in the 90's: fighting games. And in the late 80's the NES comes along and continues to revolutionize gaming again over and over, because they are trying to make unique games that are made specifically for the home.
The 80's had the fastest evolution in gaming. In 1985, Super Mario Bros comes along and blows everyone's mind. The game is just so awesome compared to what we've played before. Three years later, Super Mario 3 comes along and totally blows it away. That is what it was like to be a gamer in the 80's. The best game ever made could be blown away just 3 years later. Imagine if Breath of the Wild was released just 3 years after Ocarina of Time. That is the kind of gaming we were getting in the 80's.