Sonic & Knuckles (Mega Drive/Genesis)
Interesting one, this. As a standalone game, Sonic & Knuckles is a lot like Sonic 3, only a bit shorter and less finished-feeling. That's because it's the rest of what Sonic 3 was supposed to be, as development time cuts forced Sonic 3 out before it was technically complete. Not that we noticed, mind.
But its masterstroke is its backwards compatibility. If you slot Sonic 3 into the top of the Sonic & Knuckles cartridge, you get the complete game – Sonic 3 as it was meant to be seen. And, sure enough, it's sensational. Vast, intricate and with so much to discover, you'll wonder how you ever failed to notice the extra content hidden in Sonic 3 before.
Above: Stages like this were never the reason we played Sonic. Too much metal makes Sonic a dull boy
Knuckles is now playable and you can use his digging ability to open up new routes in the old Sonic 3 levels. And yes, they were there all along!
The 3D globe bonus stages return (boo) and there are some extra, standard bonus levels, some of which hurt to look at due to their garish colour palette. But with an enhanced storyline, Super and Hyper Sonic to unlock, Sonic 3 and Knuckles is probably as far as Sonic can go in 2D.
If you finish it with both sets of emeralds (The first lot are cancelled out as you hit the second half of the game, forcing you to collect more), you will feel that you've experienced something rare and special. But the dodgy bonus stages and the smattering of uninspiring levels still grate. So we can't say it's as timeless as the first two. Shame.
So there we have it - all the old 2D Sonics. Most of the 16-bit games are available for current machines - XBLA on Xbox 360 and Virtual Console on Wii - if you don't have the old consoles, although a Genesis/MegaDrive won't cost you the world to buy second-hand. There are also several compilations, with Sonic Mega Collection on PS2/Gamecube offering the Mega Drive titles, while the Game Gear games can be found on Sonic Gems Collection (multi) and Sonic Adventure DX (Gamecube). There's also Sonic Jam on Saturn, which lets you play Sonic 3 & Knuckles, so that's definitely worth a look too.
Check back tomorrow when we'll be continuing our Sonic coverage with a run-down of the hog's best moments - complete with an awesome video. In the mean-time, why not read up on the rise, fall and deafening crash of Sonic or take a look at this collection of funny/strange official artwork.