Could you say "Niets is meer belangrijk dan vriendschap"
I don't think so. I don't think they form superlatives that way ever.
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.
Why can't you say "Er is niets belangrijker dan vriendschap"?