For me, it's Mythras, and it's Mythras because I can steal ideas from literally everything else as needed. It's easy to say "this game does everything I need" but in the case of Mythras, I see it as the sanest "base system" around, and it's structured in a way that actually lets me incorporate mechanics I like from elsewhere without any problems.

Mythras was originally RuneQuest 6e, but after the authors lost the right to use the RuneQuest name, they transformed it into a thoroughly generic game. It's really a toolset for making RPGs, like GURPS, but it provides way more sane defaults, is way less confusing (IMO) and doesn't do GURPS's "be universal by making a few hundred different sourcebooks" thing.

So at this point it's basically a fork of BRP (RuneQuest, Call of Cthulhu, etc.), and personally I think it's a direct improvement over it.

I like that Mythras is heavily focused on everything making narrative sense. Something happens because it happens in-universe, no characters getting new magical abilities or "feats" that other people somehow can't do without them actually learning how to do those things in the story you're telling. But it manages to do that while still feeling like a traditional stat-y, game-y RPG, rather than an improvisational storytelling tool like a lot of other modern games.

I think BRP style skill resolution is good because it's boring. How good are you at Athletics? You've got 30 points in it, that's a 30% chance of success, straight-up. It's extremely clear. As a GM, I think it makes deciding the difficulty of checks (you're really modifying how many points the players have in that skill) very easy compared to other systems. d% for the win.

I think Mythras' combat system is the best in the industry, if you're looking for a game that avoids abstraction. I know exactly what a fight in Mythras looks like without having to ask players to "describe how they swing their sword" or whatever because it's mechanically telling you that already. There's no need to argue over what HP actually is or what really happens when a character misses. Mythras manages to do that without getting painfully slow, actually it feels faster than D&D combat, because most combat ends quickly. A defender can take advantage of an attacker's mistakes, getting stabbed is about as bad as you'd expect getting stabbed to be, and you can end up in states where someone should surrender after a single round or two. You're given tons of options in melee combat other than "I swing my sword" but they're gated by successful rolls, your equipment and the situation in a way that stops choice paralysis from happening too often. Your setting involves guns? Mythras Firearms has you covered and those are some of the best rules for modern combat I've seen, too.

Mythras is crunchy, but it's not meandering like D&D is. I don't feel like there's always another weird rule exception that people could end up arguing over on a YouTube video. You learn how to play the game and that's sort of just it! Mechanics all work similarly enough that I find them much easier to remember.

Mythras has rules for everything that matters. Wish you had more structure for social encounters rather than everything other than combat being delegated to a single skill check? The Mythras Companion has you covered (if the core book didn't already). Want your players to be part of a guild, religious organization, something that they'll advance through, rely upon and be answerable to? Mythras has a huge chapter Cults, Guilds and Organizations that provides more narrative (but still structured) ways to do something like a class system.

Speaking of, Mythras is classless, being BRP-based. As presented in the core rulebook your character is defined mainly by their culture and their profession, which I love. Characters actually come across as real people with significant lives outside of "adventuring" or whatever. It's a wonderful tool for introducing players to the world you're dropping them into. Being "good at fighting" isn't something that needs to come at the exclusion of being good at other things, not even being a magician of some kind, but you don't need to be good at fighting. "Combat styles" are an awesomely flexible way of handling it.

Magic is a totally optional part of Mythras, you super don't need any in your game for it to be fun, but damn does Mythras have magic rules. Actually, it has five magic systems and encourages you to heavily tailor them to your world. Those systems vary significantly in how much power they give your players.
Folk Magic is something you can give your players without worrying about balance much at all. It's all little domestic spells people might use to heat their home, or fix something that snapped apart.
Mysticism is "monk magic". It's themed around inner strength and includes lots of stat buffs, alongside abilities like wall climbing and fitting through impossible spaces. You could use mysticism to have whole campaigns of less "grounded" characters flinging giant swords around like Guts, if you wanted.
Animism is all about the spirit world. Want to play Supernatural as an RPG? you could do that with Animism, easily, although a lot of the chapter is about the shamanistic social structures that usually surround it as a practice.
Theism is what it sounds like, you're getting magic from a god or gods. This is the most conventionally balanced magic system in Mythras because it makes narrative sense for "more powerful" miracles to only be available to extremely faithful people, or people of a high rank in the local cult. The book Classic Fantasy, which is a Mythras rewrite of classic D&D, uses the mechanics of Theism as a basis for traditional D&D magic.
Sorcery is the magic system that leads people to call Mythras a Sword & Sorcery game, because yeah, the defaults Mythas gives you are perfect for that. Sorcery is the closest aesthetically to the classic D&D wizard trope, but it's terrifyingly powerful. You can "shape" sorcery spells to be more or less powerful at will, and combine them to do multiple things at once. Sorcery's primary "just hurt things" spell is called Wrack and unless you modify it as a GM, your player will be an armor-ignoring murder firehose for at least a full minute. Smart players who really dedicate themselves to sorcery could become immortal litches, or perform assassinations by calling down lightning bolts on people while they're sleeping (right from the comfort of their creepy wizard tower). I find it very cool how Sorcery lets things like that happen systemically, but it's also so overpowering that the developers warn you in the rulebook about it. Which... yeah.

Examples of the flexibility of Mythras include:
M-Space
Odd Soot
Perceforest
The Mythic Earth series
Destined (yes, it's a superhero RPG)
Classic Fantasy
Worlds United
After the Vampire Wars
Firocetta
And the list goes on.