Most people will tell you Archive of Our Own, but I keep finding the student-teacher dynamics there get really heavy, really fast. Not that they're bad, but sometimes you just want something that doesn't feel like a dissertation on power dynamics, you know? I've had surprisingly good luck with Quotev, weirdly enough. The tagging is chaotic, but the writers seem to skew a bit younger and the stories often have this raw, dramatic energy—think dramatic hallway confrontations and secret notes, less brooding philosophical angst. Wattpad's algorithm will feed you a ton of them if you search the right tags, but quality is a huge gamble. You'll sift through a hundred 'I'm your new stepbrother AND your teacher' clones to find one where the characters actually sound like people.
What defines 'best' is so subjective. If you want meticulously crafted, psychologically dense stories that really sit with the moral discomfort, AO3 is unbeatable. But if your mood is more 'forbidden romance with a side of classroom daydreaming,' the smaller, less curated spaces can hit that spot. I sometimes browse Fanfiction.net out of nostalgia, and the classic fics from the early 2000s have a specific, almost quaint tone to the trope that's completely different from modern takes. The platform itself is a pain to navigate, though.
The real secret is less about the platform and more about following authors who handle the trope well, wherever they post. I found my favorite writer for this dynamic on Tumblr of all places; she posts snippets and links to her AO3. So my advice is always to find a story you love, check the author's profile, and see where else they're active. Community circles matter more than the website banner at the top.