Boy, a lot of good answers here.
At various times, my answer would be Castles & Crusades, Beyond the Wall (and related games Grizzled Heroes and Through Sunken Lands), Shadowdark and Pirate Borg.
C&C, although hampered by a weird resolution system that's actually worse than the default d20 resolution system popularized by 3E, works seamlessly with all TSR D&D materials and even 3E D&D stuff, meaning that it's a great replacement system to use if you want something that feels more like TSR D&D, but without some of the clunkier outliers. It's also got a wonderful class-and-a-half multiclassing system that I would love to see other RPGs adopt. That said, it's SIEGE resolution system is awful -- we ran C&C online for years and we had to regularly re-explain the system, which is a real issue. And C&C books have the worst editing you can imagine (at one point, they had identically named, but different, monsters in two different bestiaries, along with numerous typos and factual errors). The Troll Lords seem like nice guys, and it's very cool that they're working on building an RPG printing press in the US to help out the rest of the industry, but the sloppiness of their books, along with the damned SIEGE engine, eventually exhausted me and I threw in the towel on them.
Beyond the Wall takes an AD&D chassis, tweaks spellcasting rules a bit and then -- their real innovation -- create fiction-first playbooks to use to roll up characters. You're not a fighter/magic-user, you're a fey-touched child. You're not a wizard, you're the daughter of the village witch. Everyone rolls their characters up together, and the playbooks have you roll on tables that knit together everyone's backgrounds together (Powered By the Apocalypse style), creating a real cohesion for the group before play begins. The playbooks also guide the group through helping create their starting village and then the DM uses a similar mechanism to run adventures more or less on the fly, based on simple tables. And these adventures are among the best I've ever run, despite being incredibly simple to generate. I was skeptical about moving over to Shadowdark, because the adventure generators in Grizzled Adventurers (about elderly adventurers, instead of young heroes) is just so good. If the folks at Flatland Games ever wanted to make their adventure generators system-neutral, they'd have a huge hit. And every RPG would benefit from playbooks like their games use to create characters and the starting town. I am over most of the 1Eisms, which can't really be defended on any grounds other than nostalgia, but the new stuff Flatland brings to the proceeding are best in class.
And then there's Shadowdark, which is all of the grit and danger I like of TSR D&D and the OSR, with the modern innovations like advantage/disadvantage and innovations that I didn't think I needed them until I tried them in play. Always on initiative and real time torches tend to ruffle feathers when people hear about them, but they work amazingly in play, solving perennial problems (at least at my tables). The game runs fast and efficient, and is the OSR game that even my 5E players are excited to play.
I like the idea of Mork Borg, but it's simply too miserable, and the book is too difficult to use, for my tastes. But it's endlessly getting remixed, and Pirate Borg, which I describe as Pirates of the Caribbean meets the Evil Dead, uses the system to create one of the most purely fun RPG experiences I've ever had. I grin so hard during adventures, my face hurts at the end of the game. Couple that with some really great adventures (Buried in the Bahamas is, for my money, the best starter adventure I've ever run, and I've run it four or five times now) and I'm really excited for the expansions shipping later this year and, beyond that, the full Dark Caribbean setting.
We live in a time of riches for fantasy RPG players and groups.