I'm not sure it's possible to get an accurate account, since every piece of software is written for different metrics. For example, the Counterize plugin seems to count each and every hit, every time. WP-Stats seems to ignore bots (counter to Monika's point). Google seems to count the least possible traffic, for whatever reason....
Example: you might see 150 hits reported from Counterize, 90 hits reported from WP-Stats, and 40 hits reported from Google. Counterize sits on your blog and notes every time your site gets "hit" - including bots and your own traffic (unless you set to ignore it). WP-Stats sits on WP's servers and relies on scripts to report traffic from your site to WP's servers, and they choose what traffic to report as a "hit" while some traffic acts in "stealth" to avoid being counted. Google relies on Java script, so any user/browser with Java disabled won't report anything to Google, combined with Google's own criteria of what constitutes a "hit" and the stealth traffic.
So, there's no perfect counter. However, WP-Stats seems to be the least biased and most accurate I've found.