If you simply mean, which languages do the most people speak (either native or by second language), then the list is as follows (according to VistaWide's estimate).

1. Mandarin Chinese: 1,051,000,000 people
2. English: 510,000,000 people
3. Hindi/Urdu: 490,000,000 people

However, if you want to know which languages are spoken by the highest percentage of places, then the list (estimated by Nations Online) is as follows:

1. English (69 countries)
2. French (42 countries)
3. Arabic (25 countries)

Your question does not seem to have an agreed answer. However, Mandarin is always listed first with Spanish and English always in the top four, with either Hindi or Begali as the possible fourth.

The Summer Institute for Linguistics (SIL) Ethnologue Survey (1999) lists the following as the top languages by population: (number of native speakers in parentheses)

1. Chinese* (937,132,000)
2. Spanish (332,000,000)
3. English (322,000,000)
4. Bengali (189,000,000)
5. Hindi/Urdu (182,000,000)
6. Arabic* (174,950,000)
7. Portuguese (170,000,000)
8. Russian (170,000,000)
9. Japanese (125,000,000)
10. German (98,000,000)
11. French* (79,572,000)

But the following list is from Dr. Bernard Comrie’s article for the Encarta Encyclopedia (1998):
(number of native speakers in parentheses)

1. Mandarin Chinese (836 million)
2. Hindi (333 million)
3. Spanish (332 million)
4. English (322 million)
5. Bengali (189 million)
6. Arabic (186 million)
7. Russian (170 million)
8. Portuguese (170 million)
9. Japanese (125 million)
10. German (98 million)
11. French (72 million)

Yes, Chinese Mandarin is the language with the largest number of native speakers. According to the source (2005), Chinese mandarin is Rank 1st , the most used language in the world in terms of native speakers. 2nd is Hindi, 3rd is Spanish. English rank 4th language with the largest number of native speakers, and rank 1st with the largest number of non-native speakers.