Killua's door
The most thickest door anyone has ever seen. It is the door to the Zoldyck's estate. This door weighs 5000+ pounds, though Killua Zoldyck can open it no problem. I'd say the door is more like a wall.
Gon: I swear I'm Killua's friend, please let me through Killua's door!
That guard: Sure
That guard: Sure