A few years back, I fell down the rabbit hole of webnovels and ended up trying what felt like every free site out there. Royal Road became my anchor. The quality varies wildly, but the sheer volume of progression fantasy and litRPG there is unmatched, and the community rankings actually help you sift through it. I’d pair it with Scribble Hub for more niche genres and ongoing translations.
That said, the definition of 'best' really depends on what you’re after. If you want completed, professionally edited works for free, your options thin out. Project Gutenberg is a classic for public domain classics, obviously. But for ongoing serials, the dedicated reader forums on sites like Royal Road are half the value—you can see what chapters people are reacting to, which helped me avoid a few stinkers that had promising blurbs.
I still check NovelUpdates daily for tracking my translated series, even though it’s an aggregator. It’s the central hub for finding which official translation site (like Wuxiaworld or Webnovel) has the latest chapter of something. Just be ready for a lot of pop-up ads on some of the host sites it links to.