If anyone could answer this question accurately then it would be valuable to online marketers. In reality it is dependent on what Google's algorithm determines is relevant out of all the data sources Google has available. Although unique to each search you can make a number of observations:
- A search for a product will bring back data from Google shopping feeds in images
- A search with a local intent (e.g.
pizza north london will bring up local business data and Google maps
- If a search can be matched against a YouTube video then that will appear prominently in the search results
- A search for a famous person will return a Google image and Google news article in the top results
It really is dependent on the query and its intent; if you are intending to optimize for competitive phrases then it is best to take this into account and try to get visibility for as much screen real estate as possible. That is even before you have taken into account paid adverts.
A good article that illustrates this and shows the impact of blended search on click through rates using heatmaps is available at SEOmoz here: