Sequels are a dime a dozen. Here are ten that really meant something.
10. Half-Life 2 (2004)
While the original Half-Life was largely responsible for creating the FPS genre as we know it today, its sequel presented a lot of variety to its action across a wide variety of scenarios, from the survival horror-tinged Ravenholm to the beachside racing segments in the remote outskirts and outposts outside City 17.
Even more significant, though, are Valve's technical merits with the game. Aside from being the first true single-player use of the powerful Source engine, the game-changing physics employed by HL2's gravity gun arguably influenced the now-undeniable presence of dynamic environments in video games rendered through real-time physics engines such as Havok an everyday occurrence.
9. Castlevania: Symphony of the Night (1997)
While Castlevania is historically rooted in arcade-style design, Symphony of the Night re-defined the series to what it is today. Borrowing heavily from Super Metroid's non-linear gameplay, Symphony dropped Dracula's son, Alucard (last seen in the NES' Castlevania III), in his dad's sprawling gothic manor on a patricidal mission with few directional indicators.
Plumbing the depths of the castle's hundreds of rooms, antechambers, and passages is to this day still an explorer's dream, while mysterious items, location names, traps and secrets suggest a rich history that's left almost entirely to the player's imagination. Toss in an action-RPG's leveling and equipment systems and an entire secret castle that could extend Symphony's length to well beyond 30-40 hours, and you've got one of the finest 2D adventure games ever.
8. Resident Evil 4 (2005)
After the 128-bit Code Veronica failed to address growing concerns over the stagnation of survival horror (and given the near-concurrent emergence of the Devil May Cry-school of game design), Capcom decided to go back to the drawing board for the long-awaited next numbered entry in their flagship horror series.
With its 3D shoulder cam, emphasis on action, and a faster, more intelligent breed of enemies that had more in common with zombies in Danny Boyle's 28 Days Later than Romero's shambling undead, RE4 was without question a revolution for the series, replacing the creep-factor with an intense battles against hordes of relentlessly pursuant foes. The tension in RE4's combat made for some nerve-rattling gameplay (there were few dull moments in its twenty-plus hour length), single-handedly changed the feel of modern survival horror and paving the way for numerous action games.
7. The Legend of Zelda: The Ocarina of Time
Much in the same vein that Mario 64 revolutionized Nintendo's mascot for a modern age, Zelda's first three-dimensional adventure was as ambitious as it was highly influential. Bringing Link into 3D presented players with an entirely new set of rules to play by, with a massive, deep Hyrule to explore across a mythic narrative that saw Zelda's Hylian hero travel across time while Link himself grew into an adult.
At the same time, Ocarina re-introduced familiar mechanics into a third-dimensional setting, adding a new layer of complexity to the series' trademark puzzle-filled dungeons. Even the concept of what constitutes modern Zelda stems from Ocarina, in everything from aesthetic to smaller innovations like auto-jumping, which wouldn't exist without this seminal entry.
6. Sonic & Knuckles (1994)
While at first the level selection may to some degree feel like a B-sides remix of Sonic 3, Sonic & Knuckles stands out (along with Sonic 2) as perhaps the most superlative example of the series' brand of platforming in spite of some familiar environments. Essentially two games in one, Sonic & Knuckles introduced branching paths to Sonic's otherwise traditional speed-based design, creating a slower and more exploratory gameplay style through Knuckles' gliding, climbing and barrier-breaking abilities.
This Genesis swan-song was also a technological marvel of its generation: the core game was highly tuned and polished, while gamers with Sonic 2 or 3 could use its piggy-backing tech to plug Knuckles into adventures previously reserved only for Sega's blue mascot, suddenly giving earlier entries an entirely new layer of gameplay to explore. Needless to say, the replay value here was pretty high.