19. The Zero Escape trilogy (Spike Chunsoft, 2009, 2011, 2016)
A trilogy of visual novels interspersed with room escape puzzles, which doesn't sound terribly exciting on paper. However, what makes it into something truly great are the merits of its writing - between these, you're presented with a colorful cast of characters and a complex science fiction tale exploring themes of time travel, humanity and morality. The former also comes into play in the overall gameplay, requiring the player to revisit earlier choices and make different ones in order to see all the possible outcomes of the story and ensure the best endings. It may not be the most technologically impressive of franchises, but fans of cerebral sci-fi will find a lot to enjoy here.
While it wasn't the first game in the franchise to utilize an open-world style of exploration (that being the relatively unpopular Castlevania II), Symphony of the Night was the game that took it to perfection. Giving the player tons of options and abilities, as well as an enormous castle environment to explore, ensured that Symphony of the Night is a widely played game that still enjoys a massive cult following to this day. While I never was a big fan of the Castlevania franchise, there's no denying that SOTN is a very worthwhile endeavor for the sheer amount of gameplay it offers - challenge runs, speedruns, or just casual playthroughs are all a blast in this one. Oh, and it's got some really kickass visuals and music to boot.
17. Thief 2: The Metal Age (Looking Glass Studios, 2000)
A followup to the original Thief, taking place a year after its events and chronicling the rise of a sinister new organization called the "Mechanists" who take the world by storm with their advanced technology. To that end, the player can expect all sorts of anachronistic fun - security cameras, turrets, complex locking mechanisms, submarines and yes, even bomb-launching security automatons. It's a little silly, but it all works surprisingly well thanks to Looking Glass' brilliant atmospheric sound design and strong character writing. It also allows for the player to utilize some clever new tricks of their own, like climbable vine arrows (which can be shot into both wooden surfaces and metal grates), exploding frogbeast eggs and "scouting orbs" utilizing the protagonist's mechanical eye to see into new areas with minimal risk. And of course, the usual stealth-based gameplay is a ton of fun and lends itself to lots of interesting mission objectives.
16. Illusion of Gaia (Quintet, 1994)
Easily my favorite Sega game of all time, as well as one of the finest JRPGs ever released to this day, Phantasy Star IV certainly went all-out to prove that the Sega Genesis could hold its own against its competitor. Not only for its great visual design and an outstanding soundtrack that perfectly highlights the game's science fantasy setting, but for its strong gameplay. Utilizing a macro system that streamlines random encounters and allows players to combine many spell effects together for greater effect (over a year before Chrono Trigger, no less*), there was a lot more on offer here than the complex maze-walking its predecessors provided. But the real kicker is the narrative, which utilized manga panel styled cutscenes to advance its story in a unique way for its era and a narrative that provided for some strong characters and a perfect wrap-up to the series' running storyline. A definite must-play for RPG fans.
14. Ultima Underworld: The Stygian Abyss (Blue Sky Productions, 1992)
A game that was built from the ground up to not just be a more realistic kind of puzzle-driven dungeon crawler, but a full blown life simulation too. To that end, you had skills not just oriented around combat and spellcasting, but for swimming, conversing, identifying items and bartering with NPCs among many others. It had a lighting system and rudimentary physics for platforming, letting objects bounce off of walls (and activate switches) and no single set solution for most puzzles, letting the player take an innovative approach to solving the game's mysteries. Downright mind-blowing stuff for 1992, and the influence it's had on the industry since is immeasurable, inspiring games like Elder Scrolls, Half-Life 2, Deus Ex, and numerous others. It's a bit clumsy and awkwardly slow to play today, but it's nevertheless a great game and an important building block for gaming as a whole.
12. Fallout: New Vegas (Obsidian Entertainment, 2010)
Fallout was one of Black Isle's big hits in the late 90s, so many fans were disappointed when the IP fell to Bethesda and became more of a futuristic Elder Scrolls than anything. New Vegas saw the original writers return, however, crafting a sprawling world more in line with the series norm while still utilizing Fallout 3's engine. The end result is what one would expect from Obsidian - well written, complex, and with a ton of content (and much more planned, as evidenced by mountains of unused content in its code). Of course, the limitations of the engine also become much more evident here, with many bizarre bugs throughout even after several patches. Still, the game is such a deep and engrossing experience that you really don't mind them too much, and the sheer number of fan mods out there lends it some tremendous replay value as well. Not to mention some very high-quality downloadable content missions that add a lot to the overall game, retaining the same sharp sense of humor and strong writing Obsidian is known for. New Vegas may not be a perfect game, but it's an immensely enjoyable one that can easily captivate you for months.
11. Mega Man X2 (Capcom, 1995)
Mega Man X2 is another example of the first game in the series introducing plenty of new elements, but the second one coming along and pushing them to perfection. Mega Man X2 is just that, pushing its stage design, visuals and music to new heights while retaining the same basic feel and format of the original Mega Man X. Of cours,e they also introduced some new elmeents to the game in the form of some clever upgrades (with the armor in particular allowing for a screen-sweeper attack once enough damage is sustained) and some new nemises in the form of the "X Hunters", who enable an alternate story branch if all three are defeated before the final fortress.