There is no specific ratio that is best.
Depends on a number of factors, including the ballpark. Contrary to popular belief, it's not always better to be a groundball pitcher. A pitcher who plays in a spacious ballpark, like in Los Angeles or Washington is probably better served by letting up fly balls. Groundballs will become hits more often in such a case. Flyballs will stay in the park.
Also, you also have to consider whether a pitcher walks a lot of batters. If so, groundballs are probably better because they induce double-plays. If a pitcher has a great strikeout-to-walk ratio, however, flyballs could be the next-to-optimal result of a pitch.
Johan Santana isn't exactly a ground-ball pitcher, but he has a great strikeout-to-walk rate and plays in a fantastic ballpark for flyballs.
In general, groundball pitchers are better for fantasy purposes. (So long as they have a healthy strikeout rate.) Not because they let up fewer hits, but rather because they let up fewer home runs. Less home runs means less disaster and damage to your ERA.