Title says it all.
I'll go first. I say, Chrollo Lucifer, simply because his ability is theoretically unlimited. He can keep stealing more abilities and keep stacking them up. Even though one of his arms stays occupied during that, his actual ability can still be used in combat, especially against opponents unaware of it.
Kurapika is the strongest that guy is beast when he get those red eyes and goo all out.
He's definitely one of the strongest. However, a good portion of his abilities are only useful against the Spiders. I wouldn't say he could beat Chrollo, Gin, or the chairman, or the ant king. Although I prefer his character and story over the others.
Netero was the strongest person EVER in his prime. Even before he died he was still one of the strongest people in the show. I stopped reading the manga the chapter before Killua obtains Godspeed. I will continue soon