They all serve a use but in Wano, it was definitely Usopp. They'd probably still win the war just fine without him though I guess Kinemon and Kiku might die so he was important in getting them to help.
Chopper saved people from Queen's plague then stalled Queen and Perospero and Nami atleast finished off Ulti so Usopp's kinda at the bottom of the list of essential members lately.