For some odd reason, even though I've heard that Arlong is stronger than Don Kreig, I believe it's the opposite…
During the Don Kreig fight, in the end Luffy could barely move and almost drown
in the Arlong fight, Luffy did not look so bad afterward and seemed to be in better shape...
http://animemedia.org/vb/showthread.php?t=1012
We've already had a fairly large thread on this.
http://ap.animemedia.org/showthread.php?t=1012&highlight=arlong+kreig
see, there's a search button.
btw, i'd say even the turtle is stronger than kreig. 
remember that luffy was fighting kreig on a battlefield bad for him: encircled with water.
EDIT: while i was attempting an answer, Battle Franky, once again, took advantage and was faster than me. :banghead:
