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. :)
/u/[email protected] is right on the money. Mana paces the game, so anything that can break that is super good. In an otherwise even matchup, if one player has a Lotus while the other doesn't, that can easily make the game. It's not going to win the game in and of itself, but it's a huge enabler to play the thing that will win you the game, before your opponent can reasonably do anything about it.
On top of that, it's literally good in all decks. It's been banned in every format besides Vintage, where it's restricted to one (and not including casual/fan formats). It had to be banned partly for power reasons, but also because it makes deck-building less diverse. There's no deck that wouldn't want a Lotus if it could have one, much less four.
It's also part of the Reserved List. After WotC overprinted cards, they essentially promised not to reprint certain ones. I think it's a dumb decision, but they've annoyingly stuck to it (and players are worse off for it). Black Lotus is on that list. And it was alreadly limited in printings, because it was a rare card, and a bit of a design mistake.
It's also simply an iconic card. Despite being a design mistake, it's a major part of Magic history, and gets referenced all the time. To some extent, it's famous for being famous. That makes it the biggest prize for collectors.
So, all this together, it has an incredibly high demand, a very limited supply, and no indication of a reprint anytime soon.
So I printed off a proxy at a professional card printer for 30¢. :)