One Piece
Very much a slow burn at first and the first couple of episodes are no help. Meeting Usopp introduces you to a quirky and unusual character and that really helps. Then you get into Nami's backstory and you're pretty much hooked. Lots (probably too many) of boss fights are very capable of shaking you off but hang in there and you'll meet Sanji: you only get a fraction of his backstory in a way that's really fun to watch and if you're not totally hooked then Chopper's backstory will do it for you to say nothing about Robin, Franky or Brook (but particularly Franky and Brook).
And just about everything between Water Seven and Sabody Archipelago is freaking amazing.
It's absurdist and funny and compelling and fun to look at and watch and it's absolutely top shelf anime. Watch the subbed version because the voice-acting is utterly perfect: can't imagine anyone else other than the original actors voicing Chopper, Frank and Brook (and you can test yourself on this because Franky's original actor retired and his replacement - I'm sure he's a well-respected guy - but he's just no Franky).
Trust me on this: drop whatever you're watching and check out One Piece. Gurren Lagan is great but this is even better. And yes Luffy can be annoying. MHA is awesome but this is better. Kill La Kill is effing awesome but this is better. The original Ninja Scroll? Awesome too but this is better. Period. When it's at its best it is probably better than anything you have ever watched: any series, any movie, any franchise.
Yes it does get long winded but even with that this is still king of anime.
Yo-hohoho, Yo-hohoho,
Binkusu No Sake Wo,
Todoke Ni Yuku Yo
Umikaze Kimakase
Namimakase
P.S. Note that I am talking about the Animated Series and not the ultra-abridged live action thing on N. Also, yeah it took a long time to watch and I hate that I spent so much time watching it and I'll never have the time to watch it all the way through again and with Franky gone I just can't get into the same way but I miss these guys like I miss family. You'll see.