It's my all-time favourite and most played game in history. I still love it after all these years!
On Sonic & Knuckles, you can play either Sonic or Knuckles. They both have their own abilities. For example, Sonic is much faster and can jump higher than Knuckles. However, Knuckles can glide and climb walls, but can't run or jump as high as Sonic. Each character has different story paths, Sonic must defeat Dr Robotnik and Knuckles must get his master emerald back from Robotnik.
The gameplay is fast no doubt about that, but it's also the most exciting and easiest in my opinion. The levels are brilliantly designed, such as the Flying Battery zone where you are on an airship, and you must go in and outside the airship to get through the level. Another level is the Sandopolis zone, where you go through a desert type of level and then through a pyramid full of dangerous ghosts and traps. My favourite has to be the Lava Reef zone though, it has some of the best Sonic music and coolest level elements in it. The bosses on that level are great as well. My 2nd favourite has to be the Death Egg zone I love that zone it's so futuristic and has some of the best hazards ever used on a Sonic game (the one that has the best has to go to Scrap Brain zone on Sonic 1 though). The other zones are Mushroom Hill zone, Hidden Palace zone, and Doomsday zone (Sonic only, and you have to unlock it).
While on the zones you might encounter some large special rings. When you go into these you go into probably the best Sonic special stage ever made. The special stage is sort of in 3D, and it's similar to Sonic 3's special stage but they're more difficult on Sonic & Knuckles. You have to collect all of the blue spheres and avoid all of the red ones, and once you get all of the blue ones you get a chaos emerald!. Once you get all of them, you turn either Super or Hyper (depending on how many emeralds you get), and you're practically invincible!. Not to mention you can just run into enemies and jump twice as high! (Hyper Sonic only). However, you need rings to stay Super/Hyper, but there's always loads of rings in the zones anyway so it's not too much of a problem anyway.
Now lets talk about the lock-on games for Sonic & Knuckles!. You see, Sonic & Knuckles uses a lock-on technology so if you combine Sonic 2 with Sonic & Knuckles, then you can play the whole of Sonic 2 with Knuckles!. You can still glide and climb walls it's really fun!. If you combine Sonic 1 with Sonic & Knuckles, you get thousands of new Sonic 3 special stages. Some of them are really hard so if you want a challenge then make sure you try these out!. Now, obviously, if you want the best game ever then you must combine Sonic 3 with Sonic & Knuckles. There's no doubt this is going to be the best Sonic experience for you, but probably the coolest as well!.
When you combine Sonic 3 with Sonic & Knuckles, you get the complete Sonic 3!. This means you'll be able to play all of the Sonic 3 and Sonic & Knuckles levels in one!. Not only that, but you can play Knuckles in all of the Sonic 3 levels and you can also play Tails in all of the Sonic & Knuckles levels as well!. Sonic can play all of the levels as well. Simply the best!.
Knuckles sometimes takes a different path in some of the older Sonic 3 zones though. For example, in Carnival Night Zone Sonic & Tails take a cannon to get to the next zone, however, Knuckles takes a teleporter instead. Many levels with Knuckles have different changes and paths, so you can simply say that playing Knuckles is a different experience and is much harder than playing as Sonic or Tails since Knuckles can't jump as high as Sonic and Tails. He also has a different storyline to Sonic & Tails so it has to make sense and changes have to be made.
The bosses in both Sonic 3 and Sonic & Knuckles are amazingly good. For example, in one of the bosses you have to fight Robotnik on a lava filled floor and the only way you can hurt him is by making the mines on the lava hit him so you've got to stay on the platforms on the lava and avoid the mines. On the almost last boss, you're on a gravity changing arena and Robotnik controls this robot to launch these spike robots to hurt you and the only way you can hurt it is by making the mines hit the bottom of the large robot so you have to change the gravity at the right time. There are plenty of memorable bosses on this game, and they're probably some of the best in any Sonic game.
The graphics in Sonic & Knuckles are superb!. It's probably the most detailed 2D sonic game I've played. Sure there's Sonic Rush and the Sonic Advance games but I still think this has the best 2D Sonic graphics. It's all clear, detailed, and colourful backgrounds. Sometimes they even move!. There's no doubt these were some of the best graphics you could have for the Mega Drive/Genesis consoles. All of the characters are quite detailed as well.
The sound in Sonic & Knuckles is great as well, not to mention the awesome music that's used in this game. I love the Lava Reef and the Flying Battery music, but I think Death Egg zone probably has the best music since it's so cool. All of the jumping noises are still there and the losing rings noise is still a welcome here, unlike the noise they used of Sonic getting hurt in the recent Sonic games. The Invincibility and the Super/Hyper music on this game is cool as well.
There's no doubt Sonic And Knuckles is a major success, and it's truely the best Sonic game ever made. It's a shame to see that the 3D Sonic games are not staying as good as this, but things change and I don't think any other Sonic game is going to reach this masterpiece. Well, it's still going to remain my favourite game, and there's always going to be an old-school feeling left inside me, thanks to the good old Sonic games.