Nami just needs a robot body to hide in, for sheer destruction she is only behind the monster 4. She is, in fact, the only member to injure a yonko. The glassest of cannons.

Franky is probably the furthest ahead after the monster 4. He was a match for pre-gear Luffy back in the day, and crushed three members of Doffy's crew (two at once, then the last while hamstringing himself.) He could likely beat one of the weaker warlords now.

I don't think Ussop is going to get there at all.