Flossing!! Definitely flossing. Waterpik is ok as a supplemental device for cleaning, but it won't get between the teeth like floss.
Water Pik. Primarily because patients are more likely to water pik daily than floss daily.
Flossing. Waterpik is good for cleaning large particles of food from around orthodontic brackets and under dental bridges. It does very little to clean the bacteria and food that gets in between teeth. Only flossing can do that. They both have there place in the regimen of oral hygiene, but flossing is better on it's own.
Flossing. Flossing is without a doubt much much better than waterpics.