It went without saying after Sonic 1's huge success that there would be a sequel, though I wouldn't get to play it until Sonic Jam for the Sega Saturn. The game went on to be more successful than Sonic 1, with many Sonic games citing this to be the best Sonic game of all time. I personally think this can safely be regarded as one of the best, though it's not my personal favourite Sonic title.
The story is once again pretty basic; Sonic sets off in search of new adventures and along the way meets a two-tailed fox named Miles 'Tails' Prower, who quickly becomes his best friend and companion. Shortly after, Robotnik invades Westside Island with the goal of once again taking over the world, this time also utilizing his new space fortress, the Death Egg. Sonic and Tails won't stand for Robotnik's nonsense, however, and set off to put a stop to his schemes.
Most of the gameplay from Sonic 1 returns here and it's pretty consistent; the game is still simple to control and feels smooth, Rings still keep you alive and there are still item boxes to help you out. However, there were a few new features added that really made the game better than Sonic 1. Sonic can now spindash, winding up in the spot for an instant burst of speed, meaning that needing to backtrack to build up momentum is no longer an issue.
The game also introduces Tails to the series as a playable character. He can do everything Sonic can do, but sadly nothing more than that. It's a shame really as Tails can be seen flying back to Soniic when he has been offscreened and it would have been nice for players to be able to take advantage of that and to differentiate between the two characters. Coupled with the fact that Tails cannot use Chaos Emeralds like Sonic can, I find playing through the game as Tails to be slightly pointless unless you're a big fan of the little fox.
The game enables you to play co-operatively for the first time ever, with the second player taking control of Tails. It can be quite fun, though the game focuses purely on Sonic, so the second player will often find themselves off-screened, with the computer taking control to catch them back up - and that can get a little old. Still, for boss fights and special stages at least, co-op is excellent. Should you not have a second player and you select Sonic and Tails together, the AI will take control of Tails and for the most part it does a competent job, though there were a few instances in boss fights where Tails may hit an enemy a split second before you do, causing you to take a hit. Those moments are few and far between though.
Special stages are completely different; there are seven of them, and each involve you heading down a half pipe structure collecting a set number of Rings while avoiding bombs. These are aesthetically more interesting than Sonic 1's special stages and can certainly be more challenging, particularly later on, and particularly if Tails is with you and being controlled by the AI, as he moves a split second slower than you do so is more likely to be hit by bombs. Granted, he can collect Rings for you as well, but it can still be rather frustrating. With one character or co-op, however, it can still be tough, but never overly so.
Collecting all the special stages' Chaos Emeralds enables Sonic to transform into Super Sonic, an invincible being that can run faster and jump higher. He's a cool bonus, but I dislike how merely jumping with fifty Rings causes Sonic to transform and I think it would have been better if there had been an alternative means of transforming, as jumping is a must in a platformer.
I like the levels in this game; while having only 2 acts per zone may make things seem shorter, there is a lot more variety in level design and aesthetics as a result, and each of the levels are a blast to play through. The design seems to mostly strike a perfect balance between speed and platforming, while many Sonic 1 levels felt slow in comparison. There were a few points in the game where I questioned level design choices, but for the most part, it's pretty solid.
I also like the bosses in the game; though a few of the early ones feature little strategy and can be beaten easily by bouncing on Robotnik's head, they do get reasonably challenging later on and are fun to fight.
Finally, there's the multiplayer, which despite it's flaws is pretty enjoyable. You get to go through a few different levels from the main game as well as special stages, racing to the end while also collecting Rings and item boxes to gain victory over your opponent. These are a lot of fun, though they are plagued with lag and the stretched graphics aren't exactly appealing to look at compared to the main game, which is one of the best looking games on the Mega Drive. The music is absolutely incredible. I cannot think of a single track I dislike, all of them are very memorable.
When it comes to Sonic games, I personally like Sonic 3 & Knuckles best of all of them, but Sonic 2 is a very close second. It took everything Sonic 1 had done well and improved on it to make an excellent game well worth owning for your Mega Drive, or on many of the other consoles which have had ports released.