How can I keep fruit salad fresh longer
There is actually no way to keep fruit salads fresh for days, but at least you can make fruit salad and eat it the the next day.

At times I also make fruit salad at keep some for the next day. The biggest problem is that some fruits like apples and bananas oxidizes and turn black very quickly, while fruits like pawpaw and watermelon goes mushy when left to long. To counteract that, add a little lemon or lime juice to your fruit salad to stop fruits from oxidizing, and keep your fruit salad in the refrigerator to keep it from going mushy. This should keep your salad "fresh" for at least the following day

On your question which fruits are the best, there really is no exact answer to that. Freshly cut fruit don't last long, either refrigerated or covered with fresh lime/lemon juice. It still stays the best to make fruit salad and consuming it as soon as possible. And why compromise and leaving out your favorite fruits because they won't last a day or two in your salad. And making fruit salad is always a pleasure