First, let's define what some of these statistics mean:

Hit

A single file request that appears in the log of a Web server. So if you browse a webpage with four images, that action will result in five hits (4 images + 1 webpage = 5 hits).

Unique Visitors

How many different people visited a website during a given period of time.

Visits

How many times those people returned.

Impressions or Page Views

The total number of pages those visitors looked at.

I'm most interested in maximizing the number of user sessions and user session length. All other statistics are valuable, but they're all variables of user session and user session length.