When I saw the post title, I was going to say One Piece. I checked out some random episodes years ago, and it was too weird just jumping in the middle (where they met Chopper at the time) so I gave up on it, and then it got too big to get into. My friends finally pressured me into picking it up, and I read the manga in about 6 months and now I'm 3/4 through the anime and love it so much after over 20 years of anime watching. It's just such a fun world and I love all the characters, good and bad alike. How it's still so exciting after all this time is amazing and it's so awesome that all the plotlines keep intersecting like it's been planned out all along.
Since we've already got that one checked off, I love Steins' Gate as a sci fi fan. Go in blind for the best experience.
Gintama is amazing, but it starts pretty slow and doesn't seem like it's going anywhere, but then you really get hooked on the characters and the world like with One Piece. Eventually though, it turns into something HUGE and it's like it's gradually evolved into something epic. It's got a ton of anime/manga references, which will make it even funnier once you've seen more shonen stuff. I love it, but it's totally not a show for everyone, but I watched it all through Covid lockdown and it was one of the best rides of my life. It's One Piece-ish, but in a very smartass way. They break the fourth wall, make fun of you for watching, there's many disgusting bits, the reuse animation on purpose, the characters are totally insane. Replace One Piece's pirates with aliens, the MC is a mix of Luffy and Zoro, and instead of searching for a treasure, they're fighting an epic space war.