I could not tell you for sure about the olive oils you tasted without testing them, but generally if it is a Bitterness, this may be from the Alkaline Minerals in it which may indicate it has more Alkaline Minerals in it !
But, if the bitterness is from Potassium Nitrate, then it would be of Poor Quality !
So, without testing to make sure the ones you tasted which was Best ?
Do you have a Refractometer to check them with ?
This may tell you which is Best, but you need to see what kind of Line is on the scale !
Is it a very straight Line or is it Fuzzy and what is the Brix of each ?
On the apple, what was the Brix and was the line straight or Fuzzy ?