Every type of lettuce is very nutritious and great for your health so be sure to eat as many types of lettuce as you can for the health benefits.
But for the most healthiest type of lettuce that contains the most nutrition that would be Arugula which has a very strong taste to it but it's really good and makes a nice salad.
Other most nutritious greens and lettuce are Kale, Romaine, Watercress, Butter Lettuce and Swiss Chard.
When opting for healthiest lettuce you should go for any type of dark greens.