Some great suggestions so far.
I also really enjoy:
Elvis Cole and Joe Pike (Robert Crais);
Myron Bolitar and Win Lockwood III (Harlan Coben);
Tony Hill and Carol Jordan (Val McDermid);
DI Zigic and DS Ferreira (Eva Dolan);
Harry Bosch and Renee Ballard (Michael Connelly - new pairing, fascinating dynamic), also Bosch and Mickey Haller when they collaborate in some books.