The best Sonic game ever. Period.
When I was younger, I played Sonic 2 at my cousin's house. It was the first game I had ever played, and I was in awe of it. The music, the multiplayer, the attitude, and, most importantly, the speed - everything about it amazed me.
For its time, Sonic 2 is a beautiful game, with detailed backgrounds, a bright array of colours, and no slowdown (except in 2-P mode, which I will get into in a moment).
The plot is simple enough. You are Sonic, the blue hedgehog of the title. You have to stop Dr. Robotnik (AKA Eggman) from taking over the world. But this time you have a sidekick, who comes in the form of Miles "Tails" Prower, a two-tailed fox. He can fly, just not in this game. That feature is instead added in the game's sequel, Sonic 3.
The gameplay is very simple. You move from left to right, jumping into enemies, performing spin-dashes, and going very fast. There is also an optional side-quest of collecting the Chaos Emeralds, which, after collecting all seven, grant you the power of turning into Super Sonic, turning you invincible at the cost of 50 rings. You also gain a speed increase, which is slightly a double-edged sword, as the controls become slightly fiddly and sensitive in this form.
The music is what you would expect from a game in the early 90's - Peppy, upbeat, and catchy. play this game for about five minutes, and you'll be humming the music for the rest of the day.
2 Player mode is the main difference from the first game, giving you the option to race your friends through 4 stages - Emerald Hill, Casino Night, Mystic Cave, and Special Stage, where the game goes semi-3D. You have two levels with the player who gets a better score winning. In the case of a tie, You get a Special Stage to decide. There are occasions of slowdown, however, especially in Mystic Cave, which slightly spoils the experience.
Overall, however, Sonic 2 is a great game, and a true classic. You can't call yourself a true gamer until you've played it.