so many good options... i'll give four

僕、実は from SHISHAMO 2 consistently gets stuck in my head. It's not a super intricate riff, but it flows well and like... it's just really good lol

バケノカワ from Polkadot Stingray's 何者 is very silly and tickles a neurodivergent nerve in my brain

Led Zeppelin's Stairway to Heaven is, memes aside, a very beautiful riff that i think deserves its popularity, even if people mostly know it for the Wayne's World gag

and i'm not normally a Weezer fan but Undone (The Sweater Song) is a very... interesting riff. like, it's weird and i like it, and i think that's kinda Weezer's thing and... it works