In an attempt to get an idea of the chances of choosing an actual future star in the draft, I looked up the history of MLB drafts from 2000 to 2015 (I stopped there because players in '16 and beyond are just beginning to make an impact). I counted all pitchers and position players chosen, and noted whether they became at least good starting regulars in the bigs.
Pitchers were drafted most often in the Top 10: out of 160 picks, 87 were pitchers... and so far, 14 can arguably be considered aces. Of those 14, only 4 were the first pitchers picked in their year, including Price, Strasburg and Cole at Number 1 overall (Mark Prior, who had one great year before getting hurt, was a Number 2).
In comparison, 21 of the 73 position players picked in the Top 10 are considered studs in the majors. My ratings are conservative; for example, in '15 I counted Bregman the lone star over other regulars like Swanson, Benintendi, Kyle Tucker, Ian Happ, Brendan Rogers.
The math (if done correctly) reveals these odds: for teams picking a pitcher in the Top 10 -- finding an ace is at 16%. For those clubs picking a position player, the odds of choosing a great regular is 29%...
Overall, the odds of drafting a future ace pitcher in 16 years of Top 10 picks is 8%, while the odds of drafting a great regular is 18%.