Current data leads me to believe rank 5 missions are the best, rank 6 are bad.
Since you can't fill up all your mission slots with rank 5, rather than going to rank 6 I suspect rank 4 will be better (no data yet, just a hunch).
If anyone wants to contribute just give me the following information
Mission Name | Credits Earned | Extra reward (Schematic, Mission, etc)
Special Note: This data is tested on companions with no boost to slicing.
http://dl.dropbox.com/u/1868678/TOR%...preadsheet.pdf
VT: http://www.virustotal.com/file-scan/...d51-1324679757
PDF of the spreadsheet. (I did not want to reveal my personal Google docs account.)