2. Sonic The Hedgehog 3 & Knuckles
Are you team Sonic 2 is the best, or team Sonic 3 is the best? While Sonic 2 is, of course, fantastic, it was with Sonic 3 (and Sonic & Knuckles) where the series truly hit its pinnacle.
Although technically released as two separate games, they can be included together on this list thanks to Sonic & Knuckles' lock-on technology. This unique hardware mechanic on the cartridge allowed the player to plug in Sonic 3 to S&K and when combined, it created one extra-long adventure that combined the two together. The game was originally meant to be released as one entry, but development constraints forced Sega to split them apart.
These two titles have absolutely everything a Sonic game needs. Great bosses, speedy sections, a banging soundtrack (rumoured to be partially created by Michael Jackson), unique level design with branching paths only available depending on who you're playing as and a level of polish and sheen not seen in the series until this point.
This was Sega at its very best; learning from the previous two mainline titles and serving up something which will forever be regarded as a true masterpiece in gaming history.
That is, of course, if you can get past Carnival Night Zone's infamous barrel of doom. Wasted hours on that as a kid, and all you need to do is hold up and down in rhythm with the barrel. Fuming.