Keyword Scout
Is this keyword worth the climb?
Demand
How quickly App Store autocomplete suggests the phrase. Fewer letters typed means more people searching.
Difficulty
How entrenched the top 10 is: rating counts, how many target the phrase in their title, how well rated they are.
Soft targets
Apps ranking in the top 10 with under 500 ratings, a rating below 4, or no update in a year. Those are the ones to pass.