King's Field - It's just so unique. There's nothing else like King's Field. Shadow Tower is similar, but different enough to set it apart. And Dark Souls, I've tried, but it's combat and level-up system are entire worlds apart. I love the emphasis on exploration. The open-world. The digging for lore. And the first-person RPG combat. The eerie soundtrack certainly helps set up the mood for the game, too.
The Elder Scrolls - Massive open-world, yet so incredibly detailed as well. The games in this series are always a breath-taking world to get lost in for months at a time as you explore, quest, and build your character inside a virtual world for you to play with. It's been too long since we've had an entry in the series.
Metroid - Moreso the 2D entries than the 3D ones, but I do enjoy those as well. My love for Metroidvania began with Super Metroid, which I still consider the greatest single game ever made. And I've loved every subsequent entry in the series (besides Hunters and Federation Force, of course).
Dungeons & Dragons Hack-n-Slash RPGS - Think stuff like Baldur's Gate: Dark Alliance, D&D Heroes, and Neverwinter Nights. I love the bash and loot style of these games. Building your character higher and higher, and finding all that rare gear to make them even more bad-ass.
The Legend of Zelda - Mostly the 2D entries, but I have to say that Breath of the Wild was pretty damn amazing, too. I've been in love with the series since the original on the NES back in the 80s. Love the high-adventure feel, the exploration, and the light puzzle-solving. And the item-usage is very cool, too, almost Metroidvania in it's implementation in opening new parts of the map.
Castlevania - Specifically the Metroidvania entries. Symphony of the Night, Aria of Sorrow, Dawn of Sorrow, Harmony of Dissonance, and Portrait of Ruin. As I've mentioned, I'm a big Metroidvania guy. And these are defining entries for the genre. The music, the atmosphere, the exploration, the character progression; it all blends beautifully.
Super Mario Bros. - The first Super Mario Bros. on NES is what made me a gamer. Countless hours spend traversing the Mushroom Kingdom as a kid. And I loved SMB 2 & 3, Super Mario World, and Super Mario Land 2 just as much as the series progressed through my childhood. I've played many Mario games since, as well. Mario is platforming perfected.
Dishonored - I'm not normally a stealth gamer. But this series is something special. The atmosphere it's industrial/steam-punk world is top of the line. You really feel like you're in a living, breathing world. And the powers, especially the way you traverse the world, is so satisfying.
Bionic Commando - Unfortunately not much of a franchise anymore. I hope one day it returns. But the unique platforming mixed with run-n-gun gameplay is done perfectly, IMO. I adore the NES entry. The 2 Game Boy entries, while not as polished, were still fun. The Xbox 360 remake of the NES entry and it's sequel were perfect examples of how to push 8-bit games into the modern age. And the Xbox 360/PS3 reboot was an underrated gem of a game.
Resident Evil - The zombie ones. Resident Evil 1, 2, 3, Zero, and Code Veronica, and their remakes. You see, I'm a HUGE horror movie fan, and zombie movies are my favorite kind of horror movies. So the zombie-centric REs are right up my fanboy alley. The atmosphere, gore, George Romero style zombies, and crazy science-created monsters feel like something over-the-top and nuts just like an 80s horror film. Love it.
I kicked around including Contra, but honestly, it's really the first NES entry I'm so in love with. The others in the series are good, don't get me wrong, but I don't have that same love for it that I do the original NES entry.