- Gmail 16000+
- Writely 15143
- Search 7675
- Maps 6197
- News 5130
- Reader 4745
- Video 4307
- Scholar 3215
- Labs 2720
- Earth 2583
Some can argue that Google.com is very well known, so it's not necessary to bookmark it. Usually the philosophy behind bookmarking is "this site seems useful for me, I'll probably use it later".
Gmail and Writely are the most popular Google services (they're still way behind search, even though not in this ranking). Google Reader's ranking is surprising and that means its redesign was pretty successful.
Not yet in the top 10, but pretty close: Blog Search, Sketchup, Book Search, Page Creator, Google Moon, and Spreadsheets.