The languages in order of greatest number of speakers: Chinese, English, Spanish, Russian, French, Hindi (a note about Hindi), Arabic, Portuguese, Bengali, Japanese, and German, but it depends on how you count them.
Counting the number of countries in which a language is spoken puts them in this order: English (115), French (35), Arabic (24), Spanish (20), Russian (16), German (9), Chinese (5), Portuguese (5), Hindustani (2), Bengali (1), Japanese (1)
On the other hand, by some estimates the most influential languages in the world are in this order: English, French, Spanish, Russian, Arabic, Chinese-Mandarin, German, Japanese, Portuguese, Hindi
For another take on that, on the U.S. State Departments Foreign Service Institute exam, extra points are given for telephone competency in these languages (in alphabetical order): Arabic-Any variety, Chinese-Mandarin, Hindi, Korean, Pashto, Persian-Afghan (Dari), Persian-Iranian (Farsi), Urdu.
Not to make your head spin, but if you are trying to choose between a couple of languages and want a little more guidance, other languages worth bonus points, although not as many, on the U.S. State Departments Foreign Service Institute exam are: Albanian, Amharic, Armenian-East, Azerbaijani, Bengali, Bulgarian, Burmese, Cambodian-Khmer, Chinese-Cantonese, Czech, Danish, Dutch/Flemish, Estonian, Finnish, French, Georgian, German, Greek, Haitian Creole, Hausa, Hebrew, Hungarian, Icelandic, Indonesian, Italian, Japanese, Kazakh, Kinyarwanda/Rwanda, Kyrgyz, Lao, Latvian, Lithuanian, Macedonian, Malay, Mongolian, Nepali/Nepalese, Norwegian, Panjabi/Punjabi, Persian -Tajiki, Pilipino/Tagalog, Polish, Portuguese, Romanian, Russian, Serbo-Croatian (All variants), Singhalese, Slovak, Slovenian, Somali, Spanish, Swahili/Kiswahili, Swedish, Tamil, Telugu, Thai, Tibetan, Turkish, Turkmen, Ukrainian, Uzbek, Vietnamese-Std.
The one that surprised me on that last list is Haitian Creole. Although I do have it. It was one of the translations that took a few months to work out.