That's a tricky question. But Google decided to reveal a part of the mystery by showing the number of page views per week for the universal gadgets - those gadgets that work in the personalized homepage, in Google Desktop or syndicated in other sites.
Fortunately, it's very easy to see the numbers for almost any gadget (but not for feeds), so here's a list of some of the most popular gadgets:
* Date & Time - 130,290,121 page views per week
* Google Calendar - 91,491,639 page views per week
* Driving Directions - 38,615,797 page views per week
* Map Search - 31,460,719 page views per week
* Wikipedia search - 27,314,972 page views per week
* Dictionary.com search - 12,520,786 page views per week
* YouTube search - 9,515,502 page views per week
* Google Reader - 3,383,720 page views per week
Google explains that: "Gadget pageview statistics are approximate only (...) and represent the number of times that the gadget was rendered, including Google Personalized Homepage, Google Pages, Blogger, Google Desktop, and across thousands of independent pages around the web."