8. Super Mario World (SNES, also on GBA & Wii VC)
Simply put: 2D platforming perfection! This is one of the few times I'll say "if you disagree, you're wrong!". Nintendo took the gameplay elements that made the NES games great and perfected them. Then they gave you a dinosaur to ride that could eat up your enemies! (Yoshi FTW btw). This is quite simply the pinnacle of 2D platformers. Nothing even comes close in terms of quality.
Other than the gameplay being perfect, it also had hundreds of diverse levels, some beautiful worlds to explore, brilliant level design and countless extras including hidden worlds with ridiculously hard challenges. I found the artwork especially enchanting (I especially liked Vanilla Dome and The Enchanted Forrest) as it really worked my imagination; I genuinely felt like I was in this amazing world where dinosaurs and plumbers could co-exist! 
I also loved how after completing certain levels the world would change whether it be the destruction of a castle or the formation of a bridge. Speaking of the world, the world map actually had you effectively going round in a circle:
If you look at my list I believe this is only one of 3 2D platformers and definately the highest. I remember when I was about 7-10 years old playing this almost non-stop with my cousins and staying up late during the Summer holidays addicted to the joys of Super Mario World. I'm sure nostalgia plays a big part of my love for this game, but it's one of the few games I keep going back to just for the sake of playing. I think I can actually do the level Yoshi's Island #2 whilst blindfoled. Give me a SNES and Super Mario World and I'll be satisfied for many years.
