Baseball Reference has "Hall of Fame"
scores for everybody in which you earn points for various playing achievements. The average Hall of Famers has a score of 100. So far the MLB draft has only produced three surefire Hall of Famers in the #1 pick (Griffey-235 points, Rodriguez-349 points, Chipper Jones-162 points) and one borderline case (Harold Baines-66 points).
Excluding the three sure things, the rest of the group picked through 1997 (to allow time for an actual career) averages a Hall of Fame score of 13.2.
Unfortunately I don't see anything similar for basketball on their site. However, a somewhat random sample of modern major leaguers produces and average of 2.57. So they're definitely pretty good at making sure that the #1 pick is better than average.