Good call! Im thinking maybe S5, S11, S22, stand out as the most competitive seasons, as in, the seasons where it was least obvious who would go out and who would make it to the finals? They probably had the most potential Top 2/3 finalists out of any season I can remember.
In most seasons, like S2, or S4, the Clay/Ruben, Carrie/Bo finales, were hardly surprising.