What makes it your favorite? Do you want to play it? If so, what's keeping you from doing it?
For me, it's Burning Wheel.
I bought it purely based on aesthetics back in 2008ish, then got the supplements, then Gold, then Gold Revised, with the Codex, and the anthology...
I blame it for my weakness for chunky, digest-sized, hardcover RPGs. :P I also like the graphic design, I like the prose (even if it's divisive), and it has both interesting lessons you can plug into other games (like "let it ride," letting success or failure stand instead of making lots of little rolls) and arcane systems that pique my interest (like the Artha cycle, which makes roleplay, metacurrency, skill rolls, and advancement all intersect). I genuinely like reading it for its own sake.
I haven't played it because... well, since it's not D&D, that immediately makes it harder to get people interested, sadly. It's also a bit daunting, given its reputation as a crunchy system. But I have a group of players interested in trying new things, and fewer other games calling for my attention, so hopefully I'll get a chance soon. :)
Ars Magica but Burning wheel is a close second. I run and play modern/sci-fi 90% of the time as I got burned out on fantasy setting but I hold a candle for running/playing a theme campaign of a Mage Guild in ars Magica or a Theives guild in Burning Wheel.
Ah, Ars Magica is another one. I found it as a teenager browsing the internet. I even downloaded the free PDF they offered and started printing it out, before realizing just how much paper that would be. :P
There is a ttrpg youtuber that did a video on Harnmaster mentioning he uses the Ars Magica magic system in that game which sounds like an absolute delight. Me, Myself & Die is the channel name. I love crunchy systems (I've been gamemastering Shadowrun since 3rd edition) and I've wanted to get into a fantasy crunchy system but stars have not aligned ... yet. I've played and ran PF2e a lot and while the three action economy is fantastic and the world-building is delightful (Golarian's kingdomds of Geb & Nex are so unique) I really don't care for Vancian magic or class systems.
Huh, I hadn't thought of that, since PF2e is mostly poised as a D&D alternative rules system, but the setting must be popular given the existence of Savage Pathfinder... I'll have to do more digging into the lore!
I've also had this thought that I was burnt out on crunchier RPGs, before reading PF2e and realizing, no, I'm just tired of fighting against a rules system. :P I'd love to get in some crunchier games, too, so long as they work properly and deliver on their design goals.
and I'll have to check out Me, Myself & Die! I think I've heard the name before.