Which is the best Digital Marketing company worldwide?

a very subjective question. There is no universally accepted standard or definition for the measurement of such. However, you can find out a top ten (in no particular order) on different rating sites.

I worked for a company which was rated7th best in India (and that is a big achievement as India's every lane has3 digital marketing agency, or so to say) but then in rating by other agencies we simply didnt feature. 

Singlegrain is also included in the list of best Digital Marketing Agencies.

According to mashable , Hubspot is one of the best Digital marketing agency . 

Thanks for your invite. To best of my knowledge it is Wieden + Kenndy.

their website address is wk.com . Good Luck