While the Sonic Franchise has truly taken off to include tons of games, including ports of the originals to Wii and other consoles, I think that the original Sonic the Hedgehog (with the Green Hills zone) is what I always think of when reminscing on this game. And, from my brief perusal of the internet on the subject, it seems that I am in good company. Because of that, and my limited experiences with some of the other games I have chosen to focus on this, the OG version. The premise of the game is that Sonic, the main protagonist, is challenge by the evil Dr Ivo Robotnic, also known as Dr. Eggman, and is forced to go through levels saving multiple animals that Dr. Robotnic has imprisoned, as well as collecting six Chaos Emeralds (later 7, a la Dragonball Z) that you can grab in semi-difficult minigames that you gain access to as you beat each level.
So, you may ask (if you haven't played) why and how has Dr. Robotnic planned on getting all of these Chaso Emeralds? By imprisoning Sonic's animal buddies in cages and giant metallic suits that essentially keep them trapped. Luckily, as you progress through the game controlling the friendly, speedy Sonic, you are able to beat "zones" in three acts at a time and save them! At the end of each zone, the player is forced to fight against Dr. Robotnic in a boss-type motorized vehicle that is specific to the theme of the zone. There are six of these battles (hence 18 levels) prior to Sonic's entry into the Final Zone.
The mechanics are pretty simple: it is a 2D side-scroller in which the character is able to "power up" and speed through difficult parts of around roundabouts to get extra rings. Sonic is able to kill bad guys, with which these levels are rife, by jumping into them or on them. You are allotted a set amount of lives which you can increase by one for each one-hundred rings that you collect throughout the game. That being said, if you collide with an enemy or obstacle (like those piece of shit spikes in the Green Hill zone) you will lose all of the rings that you've collected; if you haven't collected any rings yet, then you will lose a life. You also lose a life if you exceed each stage's 10-minute limit. This, I guess, isn't really such a big deal now that I look at how fast the game actually moves, but when I was younger and I wanted to get ALL the rings (kind of like I still do in Mario) that limitation was super annoying. Oh, also, the special stages that I mentioned before only happen if you get to the end of the stage holding 50 rings—then you get to jump through a HUGE ring and go to the special (pain in the ass) level that I mentioned above. At the end of the game, you should have collected all six of the chaos emeralds, otherwise Robtonic taunts you—super annoying and discouraging as a child.
Ultimately, Sonic wasn't that groundbreaking—I don't think that's why I like it. Sega chose to have this game developed because they needed a "mascot" that they could turn into a brand champion, and in that they found Sonic. There are so few people in the world that don't know who Sonic is that it is a travesty I've taken this long to actually write about this game. The levels are quick, gameplay is pretty simplistic, and (all things considered) it is basically like a basic version of Mario Kart. You know what you're getting, you know what you have to do, and you basically get the mechanics—with those tools you can play on autopilot like you would Cut the Rope of some kind of iOS game. In the end, Sonic might not be a feat of excellence, but it is still a game that will always be remembered as one foundational and one-of-a-kind.
You can play Sonic online here. Happy Gaming!