Personally - I would use DOMReady. I would not trust window loaded. Window loaded fires AFTER all the images and other dependent assets are loaded. So if there is just one asset which is performing slowly - the event may never fire. (Which could yield analytics data loss)
@1982luca For triggering the Rule as late as possible you can select eventType as Window loaded, as this is the event which triggers once the page has been completed loaded. For reference sharing the snapshot of other eventType informations.