I had this debate late yesterday evening. I'd cast a vote for (not surprisingly) Maggie Gyllenhaal and Peter Sarsgaard, two of the finest actors around.
I suppose one could make arguments for Annette Bening and Warren Beatty (yawn), William H. Macy and Felicity Huffman, or even Heath Ledger and Michelle Williams. I dare you to throw Tom Cruise and Katie Holmes on the list.
Got any others?