This is a tricky question - it's not a matter of fastest export - the question is do you need to restructure the data during the export/import?
If you didn't, then you could use some sort of a tool to dump out JSON documents from Couchbase and load them into MongoDB using 'mongoimport' - unfortunately I don't see any evidence that there is a tool to dump out json from Couchbase.
Since you might want to re-consider your schema anyway (since you're basically moving from key-value structure to document structure where you can index various secondary fields directly) it might make sense to write a light weight script which pulls data from one, applies whatever transformations you need to make and inserts them into the other.
Asya