Sonic 3 & Knuckles, probably my most favourite Sonic game on the Genesis. Scratch that, favourite Sonic game ever. I began playing it when I was young, my first Genesis game in fact. My Dad showed me how to play and I was hooked since then. We only had Sonic & Knuckles, though, another reason why this game is so special.
Using lock-on technology, unheard of at the time, and to my knowledge it's one of a kind, adding on Sonic 3 creates the Megadrive Masterpiece. Sonic 3 & Knuckles. I remember when I was about 8 or 9 when my Dad decided to buy Sonic 3 off of the internet. The addition of saving was a necessity for this game with a total of 14 Zones!
 
Sonic 3 & Knuckles is my favourite for a reason, it's as great as you want it to be. For one person, the game may just be another in the Trilogy, similar to the rest, another Megadrive Sonic game. For me, I see it as a grand adventure. There's a sense of meaning, story, tension, in this one to me. Sure, it may seem cliche, but let me explain what I mean.

Here's the story.
Eggman steals the Chaos Emeralds, okay, normal Sonic story. Except it's not. We see an immediate consequence of this right at the start of the game. Sonic and Tails both crash into Angel Island, normally floating in the sky but now floating in the Ocean. Will it float forever? Like any good Heroes, we won't wait too long to find out. Eggman manipulates a Red Echidna, Knuckles that Sonic and Tails had the Emeralds all along. And, of course they did, but not for the reasons of evil. Then, the chase begins.
This may seem like a simple story, and it is. It's what works here. Sonic doesn't need to be deep, it's okay to be simple. But what I love about this is how it portrays itself as more than just... That.
I may need to explain again.
As you go through the game, you can see Eggman's effects on the environments you pass through. Burning down the Rainforest on Angel Island, an attempt at launching the Death Egg at his Launch Base, what I presume used to be part of Ice Cap Zone.
Once you get through to Lava Reef Act 2, though, is when you get this feeling of urgency. I need to stop this Mad Doctor and foil his plans. He can't win, he won't win, he's Eggman. And Sky Sanctuary, racing the Death Egg, running as fast as your Legs or Propeller Tails will take you.
This sense of encouragement done all without speech or text. All of it through just playing the game. It's what makes this Story special.

The game's Graphics are a MAJOR step up from the past 2. For starters, the environments feel alive. Backgrounds, Foregrounds, Platforms, all exploding with colour. The shifting sands of Sandopolis, the dense forest in Mushroom Hill and the intense altitude of Flying Battery.
Environments that scream imagination like Lava Reef and Hydrocity. Both Zones break the norm of typical Lava and Water levels. New Ideas like Endless Mine, Carnival Night, Sky Sanctuary, Hidden Palace, Death Egg.
One thing about the stages is the transitions. Each stage shows how you get to the next. After Angel Island you fall into Hyrdocity, after Hydrocity you blast into Marble Garden, after Marble Garden you fly to Carnival Night and so on. I really love this idea, it's what gave me the idea of this game being a 'Grand Adventure.'

As I have said, with a Total of 14 zones and 4 Multiplayer ones on top of that this game is big. It may not be the longest game in the world, but it didn't have to be. 2 different stories that link together beautifully, showing how the greatest friendships evolve from a place of conflict, then into something richer.

All in all, I think I've gushed enough. I think I've said enough to show how much I love this game and how much artistic simplicity can go. This game is great, and if you haven't played it by now... C'mon already!