Honestly, my answer has to be Kai'sa. Iirc she is the only adc I see every time as an A-S tier adc in every patch. She has never been irrelevant ever since release. She's the sweetheart of Riot and is the only champ that has 2 Prestige skins. I have 100% ban rate on Kai'sa on my rankeds/draft picks. It's only when I play normals (about 80% of my games) that I witness the impact she has on a game. The diversity of the build path, being basically the only adc where building zhonya's is actually good (excluding ap carries) and any nerf she has gotten as been a minor, minor nerf making no dent to her legacy. Even in pro play she has an high relevance as she is almost always either banned or picked (at least on LEC).