As a sales person what is your best answer if a customer ask you which is the best between your product and another competitive product ?

Say the truth, because one way or the other they are going to find out. And with help of internet and the easily accessible information available, it's not that hard for the customer to find out the exact story. Stress more importance on the USP of your product, but don't downgrade competitors product.

Present the qualities of your product and why you believe they are better. "The competition prefers to [xxx], while we, on the other hand, do it like [xxx]. We find several key benefits to our approach, which are [list of benefits]...."

Many Sales people makes mistake of downgrading competition and it mostly playes against them so always try to give how your product is better than others and dont say anytime competitors product is nothing or really bad. 

Just holding on to positive points or USP of your product makes you sell it better.