Could you say "Niets is meer belangrijk dan vriendschap"
I don't think so. I don't think they form superlatives that way ever.
I would prefer a word like "importanter" existed!
Get it? It be easier...
Liefde is ook zeer belangrijk :-)
"Niks is belangrijker dan vriendschap" - Is there a difference between niets and niks?
"Niks" is informal and should not be used in official text. It should probably be accepted here, though.