This is a question that we are asked often. In general, olive-y and peppery oils are the best all-around oils for salads of mixed greens. Use delicate and mild oils for salads made of tender greens such as Bibb lettuces, and leafy green and grassy oils for strong greens such as spinach, arugula, and watercress. Fruity and fragrant oils are the best choice for salads that have fruit as a main ingredient, such as oranges, apples, strawberries, mangoes, or sweet tomatoes.
This is just a general guide, so let your palate be your guide.