There really is no 'Best Chai' out there.
There are a lot of different companies that produce different styles of chai. Traditional (spicy) or not so traditional (sweet). Powder or liquid, you can even create your own chai in store by steeping a black tea and adding your own milks and spices.
Most of the chai companies would be happy to send you a sample so that you can taste it and try it with your customers.
I personally lean toward the liquid chai's that have a kick to them.
Try Third Street Chai, Sattwa or The Chai Co.