Personally, I like to juice up several different kinds of fruit and vegetables - which may include various combinations of bananas, red bell peppers, apples, carrots, celery, broccoli, spinach, parsley, tomatoes, cucumbers, etc.
Be sure to buy organic versions of the 'dirty dozen:' the fruits and vegetables that, when grown conventionally, are loaded with pesticides and chemicals: Grapes, apples, lettuce, bell peppers, carrots, nectarines, peaches, strawberries, pears, kale, and celery.