I was hoping for more 'pick up' on this one, given we have loads of De Niro / Scorcese fans on here. Oh well. Will go with my pick, John Carpenter & Kurt Russell.
They (to date) have made 5 films together and have clearly, a very strong friendship and it's vital for any actor to trust one's director and vice versa. The journey you take together is hugely dependent on you getting along and believing in the material.
For me, the greatest of the collaborations, is The Thing. Russell is at the top of his game and Carpenter directs with a verve and deftness not seen in his later work.
The film (and their work together) signifies an understanding of character, rhythm and an enjoyment of each other's talent. When you have this kind of synergy, you make better work and Russell has always (in my view) done his best work with Carpenter.