There are quite a few Lexa and Clarke lemon fanfictions out there. For example, 'Intoxicating Love' is a story where their relationship unfolds in a very organic way. The lemon parts are not just thrown in for the sake of it but are an integral part of their emotional journey. Then there's 'Fated Encounters' which plays with the idea of destiny in their relationship. The writing is really good, and it makes for an exciting read. Also, 'Enchanted Desires' is a fanfiction that has a great balance between plot and the more romantic and, yes, lemon - ish parts.
Well, I know 'Burning Love' is a Lexa and Clarke lemon fanfiction that stands out. The author does an amazing job of building up the tension between them before the more, uh, spicy parts. There's also 'Midnight Encounters' which is set in a really interesting post - apocalyptic backdrop that adds to the allure of their relationship. And don't forget about 'Tempting Fate', which has some really well - written scenes that are both passionate and in - character for Lexa and Clarke.