We all know the are less and more useful characters and some of them are useful more to the player than to the team.
So which character is most useful for group? In my opinion it's Wendy. True spiders killer, can obtain insane amounts of silk and monster mean in no time, receiving practically no damage. Silk is super important for many things and monster meat.. this doesn't even need explanation.
I was torn between Wendy and Wickerbottom, but librarian weaknesses are really nasty (at least in my playstyle).
There are other characters that if used properly, can provide various of different benefits to their group:
Wigfrid - Powerful helmets and spear, I only wish more Wigfrids would realize that everyone want their gear
Wilson - Meat Effigy, although it's less useful in multiplayer
Woodie - Wood! But I am still not sure who is more efficient at getting wood, Woodie or Maxwell
Wickerbottom - I think she is the only character that cannot be blamed for staying in base. True base manager.