Western Languages by number of native speakers I'd guess.
The whole list (sorry for the digression but..) here:
http://en.wikipedia.org/wiki/Ethnologue_list_of_most_spoken_languagesBy native speakers, Mandarin comes first followed by Spanish, then English and Arabic.
The list by total number of speakers, counting those who learn them as second languages is here:
http://en.wikipedia.org/wiki/List_of_languages_by_total_number_of_speakersWith English first, followed by Chinese and with Hindi, Spanish and French after that.
There is a rather complex equation that gives languages a "cultural weight," too, based on total number of speakers, spread of the language, average GDP per capita of speakers, assessed cultural importance in the world of the countries where the majority speaks it, export/import balance, etc. It may be available somewhere on the Internet.
From what I remember, #1 was English by far, but steady (that is, its cultural importance remains the same).
#2 was Chinese, on the rise.
#3 was Spanish, also on the rise.
#4 was French, descending.
#5 was Russian, steady.
#6 was German, descending.
#7 was Hindi, on the rise.
#8 was Arabic, also on the rise.
By the end of this century, if the trend continues, Chinese would have overtaken English as #1, with Spanish and Hindi fighting it for #3 and Arabic comfortably ar #5.