It's true that Nisekoi isn't exactly original when compared to the other harems and romances, but there is something that's unique about it. Yes, it's nonsensical at times and it can get kind of silly but that's part of what i enjoy, I like the way the art shifts depending on how the characters feel and react.
The story is simple but I like the way it develops, especially since we don't really know which one of the 3 out of 4 girls is actually the girl that Raku made a promise with (though, as it normally happens in harems, the writer found a
way to self that plot for the moment) and I also like how the other members of his harem are introduced gradually rather than in one fell swoop, with the harem being completed almost 40 chapters into the story.
The characters have great personalities that aren't just simple cliches, they each have a backstory and attitude that you will like (or hate, that's your choice, I like them all!) as well as having their own silly quirks that'll make you laugh.
But it isn't just the characters in the harem but the ones out of it as well, a lot of other characters have their moments of glory and funny that at times makes them better than the mains themselves.
I liked Naoki's art since Double Arts but I really like Nisekoi because of how fluent it is, I also like all the different art shifts that he makes which can help the mood of the chapters very very much.