has:nouserlabels -in:Sent -in:Chat -in:Inbox
This works for everyone... You may want also to include -in:Draft:
has:nouserlabels -in:Sent -in:Chat -in:Inbox -in:Draft
Sometimes you will see conversations with labels with this search. But it is OK.
Google keeps labels with separate messages rather than with conversations. Therefore if there is a conversation with some message(s) without any label and some message(s) with label(s) - this convesation will still show up in the search, showing all the labels used in the messages in the conversation. Thus, you may see Inbox label for some conversation. But when you choose it, Move to Inbox button shows up. Which means that there are some messages in the conversation that have been archived. When you press this button - all the messages are labeled back to Inbox and the message disappears from the search output.
In this case you may want to operate on message level rather than on conversation level. However I have not found yet how to do it with the new gmail design. When you go into conversation, it shows all the labels above the conversation at once and does not allow to see/change labeling of each particular message in the conversation.
Appreciate any advice on this last point...