Iceberg lettuce is the easiest type of lettuce to digest.
However Iceberg lettuce is not as nutritious as other lettuce is although iceberg lettuce is still pretty healthy for you and if you have trouble digesting other types of lettuce but still want to eat lettuce then iceberg lettuce would be the best.
If a lot of types of lettuce are hard for your body to digest then you could have digestive issues that you should see a doctor about.
Some people have trouble digesting certain types of lettuce or other leafy vegetables and if so a doctors visit would be recommended to find out why you're having trouble digesting those foods.
Some of those digestive issues are things like gastrointestinal issues like IBS.