When a supplier is sitting in front of you for negotiation, which is the best way? A "win win situation" or " a win lose situation"?

There are2 kinds of negotiation - Bargaining = win-lose.  and there is Joint-Problem-Solving Negotiation which is a WIN-WIN. 

Your supplier is your partner in the succcess or failure of your business. 

The best way is the JPS. , for a long fruitful collaboration and partnership. 

Otherwise, if you use win-lose, you may succeed, but of course, you will end up doing a ONE-TIME deal. 

And dont forget your BATNA - Best alternative to negotiated agreement.. and of course, keep in mind the JPS at the same time.  

:-)