I use Google.com as my homepage.
I use Gmail.
I am a Google Voice as my browser.
I also use Google Desktop Search with the Google Sidebar. I have Google Voice in my sidebar as well..
The only reason I stopped using Google Toolbar is I felt it conflicted with IE8.
I use Google Webmaster tools (sitemaps) to promote my websites and have used Google Adwords and Google Adsense at times as well. I also use Google Analytics to track traffic to my sites.
I have a business registered with Google Local.

I've never had a need for any of the "Google Apps' office programs as the small companies I work for all prefer MS Office 2007.

As for how much does Google know about me?

I am the exception. If people cannot find me, I do not work. I use Google to promote me and my businesses. I like Google Voice and am not that concerned about privacy issue but would never say anything indictable over a telephone to begin with.
I'm sure Google knows more about me than the federal government does.

I feel to choose one Google product would be a difficult choice.

Gmail would be a good choice, but the one getting the most attention right now is Google Voice. Imagine never missing a call as all your phones ring at once (like in Lawnmower Man), having voice mail transcribed and sent in email, being able to record calls and more..

Google is so embedded with what I do daily, it would be a difficult choice to choose just one..

I use gmail as my main email account. I use google docs, so i can save my documents and then open and print them at school, because I dont have a jump drive or printer. It sucks that I'm on a mac, because I havnt got to try chrome or google voice. I have igoogle as my homepage, and google news is always the first place I go for my news, since it has articles from a variety of sources. I've not found google scholar to be that helpful, i've used it for school a lot recently. Google I'm sure knows a lot about me, since i have a profile on Orkut, and recently been using google health.