I can't actually think of any antagonists I loved, other than one and he's far from my favourite.
As people have said Blackbeard from One Piece, though I'm surprised at the reason people have given. First off the way he's introduced was as a COMEDIC foil to the protagonist, with the two having literal opposite tastes in all food and drink (for example, they both eat a cherry pie, the protagonist hates it, Blackbeard loves it). Later, after a bar room brawl that leaves the protagonist bloodied and beaten and his dreams laughed at and spat on, he gives him a rousing speech about how those who laugh at the dreams of others are fools and how the age of dreams will never end. He later tries to capture the protagonist after seeing his bounty rise to use him as a bargaining chip, though through a quirk of fate they never meet after this.
In his next appearance he defeats and important character and uses HIM to rise in power, all to break into a prison to free specific inmates for his crew. (it's more complicated than that, I'm simplifying)
Basically, in a series where dreams are a running theme, he is the first to share the same dream as the protagonist, and served to show the darker side. Against the protagonist's impulsive and rash nature, he plans and waits for years on end for the right moment. He's willing to throw others to the fire to advance his plans, unlike the protagonist.
And yet at the same time they're both wide eyed, grinning idiot optimists. They both place dreams as unbelievably valuable things. They both inspire immense loyalty in crews that could no doubt never stay together without their captain.
TL
R, Blackbeard shares most of the core beliefs of the protagonist and the series as a whole, while also showing the selfish sides of them.
Ooo got another, Crocodile from the same series. Scheming for years manipulating politics of a country to make himself ruler and find a powerful weapon, he's already a skilled manipulator clearly. But what makes me really like him is his final line before his defeat.
"The more you help people... The more you'll lose sight of your own goals!" while lunging at the protagonist. For me at least, this changed the tone of the character completely. He wasn't just an evil, selfish manipulator. He was a jaded man who tried to protect too much and failed. He could have been just like the protagonist at one point. To me, that line changed him from a good antagonist into a great antagonist, a glimpse of a grim future the could follow the protagonist's good intentions.
Whew, I DID NOT intend for such a long post!
