Damn this is really a tough one...
Let start with round one:
R1: Which one is better?
I love both arcs but If I need to decide I would take Chimera ant Arc above Yorknew Arc, for me Chimera ant arc is probably one of the best arcs if not in the top 3 in all of time anime series. Even if I enjoy Yorknew Arc more, Chimera arc was better.
R2: Which one do you like more?
Yorknew Arc, I just love the setting and the story around the spiders, the arc was brutal but also funny and amazing, Chimera ant arc is also great but I kinda love Yorknew Arc more.
R3: Better antagonists?
Almost equal I would say, both had great characters, who been also great antagonists, Chrollo and Meruem been top and I think both arcs had great anatagonists, would say here a draw, but since I need to take one I would give it to Meruem and his Royal Knights, these guys been monsters and dangerous for sure.
R4: Which antagonists do you like more?
Meruem and his Royal knights, Meruem character development was one of the best one in entire manga/anime series for me, kinda liked them more, spiders also are great but nothing tops Meruem.