The idea of a 'top' list for something like Reylo fanfiction is tricky because it depends so much on what you're looking for. Are you after a canon-divergent fix-it for the sequel trilogy, a modern university AU, or a fantasy epic? The community's favorites shift constantly. If you're just starting, you'll probably see 'Landscape with a Blur of Conquerors' and 'Force Destiny' mentioned everywhere—they're massive, epic-scale works that a lot of people consider foundational. They're good for understanding the scope of what's possible in the fandom.
Personally, I found my way in through shorter, moodier pieces. 'Reylo in D Minor' by blueenvelopes935 on AO3 isn't a book-length saga, but the prose has this sharp, almost painful quality that really captures their dynamic for me. It's less about plot and more about atmosphere. Sometimes diving into the 300k-word monsters right away can be overwhelming, so maybe poke around the 'Reylo' tag on Archive of Our Own, sort by kudos or bookmarks, and read a few summaries. You'll quickly get a feel for the subgenres that call to you. My own bookmark list is a chaotic mix of fluff, angst, and smut that I'd never claim as definitive.