Tamil, by a huge margin.
Among all dravidian languages, its Tamil which is spoken most by people of other dravidian languages and people of other Indian languages.
Looking worldwide, apart from Tamilians in India, countries like Malasia, Singapore, Srilanka & Mauritius have Tamilians.
Also Tamil is a National / Admin language in these countries.
Telugu is the most spoken dravidian language in india and so many people mistook with hindi as the most spoken dravidian language!! But it is not so, though hindi is the most spoken language in india but it is not dravidian, it is Indo-Aryan(some say Indo-Persian) If there are any grammatical mistakes in my answer, sorry for those!
Tamil is considered as the oldest language amongst the Dravidian languages.
Of or pertaining to the Tamils, or to their language., One of a Dravidian race of men native of Northern Ceylon and Southern India., The Tamil language, the most important of the Dravidian languages. See Dravidian, a.
Tamil. Among all Dravidian languages, it is Tamil which is most followed & used for communication by people of other languages, by a big margin. Apart from Tamilnadu, in Major cities of other south indian states, Tamil is the most follwed & used language, for communication, next to that state's official language and Hindi. In other parts of our country too, Tamil is the most known Dravidian language. In countries like Singapore, Malasia, Srilanka, Mauritius and even in Japan Tamil is the most used Indian Language. Tamil is an official language in Singapore & Srilanka. Tamil is the only Indian language which is an official language in other countries.
Shona is the most spoken native language, but English is a widely spoken second language for many people.
Mandarin is the most widely spoken language in the world.
Spanish
Language spoken by the most people is chinese, because China has the largest population
Mandarin Chinese.
Mandraian
Spanish
Probably Kayardil, which is spoken by less than 10 people. It is a language spoken in Queensland, Australia.