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Usually overshadowed by its spiritual successor ―Dark Souls― Demon's Souls is perhaps the most important game From Software has ever created. Sure, the DNA of mysterious and punishing dark fantasy world was already taking shape in their previous King's Fields series, but Demon's Souls was the cornerstone for every "souls" game. For once, it features some of the best level design ever put in a videogame, with open spaces mixed with intrincate corridors, traps, secrets, verticality, shortcuts... It patented the brilliant online hint system managed by the players, the anonymous co-op and PVP integrated into the game's lore with invocations and rewards for helping/killing others. And of course, the deliberate combat and difficult bosses that are challenging but can be beaten if you pay enough attention. It doesn't have the fully interconnected nature of Dark Souls but it was well structured with The Nexus as a hub to connect every world via the archstones. Each world was unique, not only in its visual design but in its level design too. Especially the Latria Tower, one of the best designed scenarios ever in any interactive medium.
Let's face it: Sonic was created by Sega as an answer to Super Mario and never quite achieved the genius in level design and addictive simplicity of Nintendo's mascot. But it was a terrific platformer with a lot of charm and style. And it was cutting-edge back in 1991. For a lot of '90s kids with a brand new Genesis/Mega Drive system, this game felt like having an arcade at home: The colorful design, the bright colors, the distinctly japanese kawaii sensibility... And of course, it had a well-thought platforming system that incorporated fun elements like speed, pinball-like contraptions, and the original way of managing death: Sonic loses all rings when damaged (but does not die), and you have a chance to recover some of those same rings.