What statistic inspires you most". My answer; it depends what hat I am wearing.
The statistic that means a lot to me are the page views. However, they do not truly show how many PEOPLE view the pages. There are the bots that spoil the view. But there is so much history there.. Even with the bots you get a picture of growth.
However, wearing another hat, I want to see an aggregation of views of GLAM objects for the GLAM's we cooperate with. This is so vital for making the point that Wikipedia is making cultural heritage visible. It is the one argument everyone "understands".
Yes, there are many languages but for this I want to know how well does MediaWiki work for a language and this can be found best in the stats at translatewiki.net.
Really Erik, there is no one statistic that is best. There are horses for courses. What is vital that the statistics can be trusted. If anything, improved reliability of the data is what would help us most.