from word in text.Split(' ') 
    group word by word into g
    let count = g.Count()
    orderby count descending 
    select g.Key

Under the covers when it is compiled it's turned into a select. That way it it only does N counts, not potentially N log N (for each comparison in the orderby), where N is the number of items you enumerate (so if you .Take() just 10 it doesn't really matter).