I voted for HT 2018, but I suppose I am a bit biased. That is what I have been feeding all my rats for quite awhile now and have had very good results. I've had all ages, from babies to oldies and all they're staple diets were 2018 lab blocks. Pregnant mothers and babies were supplemented with extra foods high in protein, calories, and vitamins. Adults were given veggies, fruit (in moderation), and a variety of other healthy foods/treats. I free feed--I use hoppers attached to the cage. I have not had any issues with adults becoming overweight or with males developing skin issues which I have heard can happen if their diet is too high in protein.
Because Trihkal's rats are females and still pretty young I am going to assume they are pretty active and energetic and would do well on HT 2018. If she did not feel comfortable with 18%, then HT 2016 would be a good choice as well. Personally I wouldn't go for a lower protein percentage than 16% for female rats unless they become overweight or there is some other medical reason.
As with many other things, everyone is going to have different opinions as to their preferred diet and there is no one "right" answer. What really matters is that you find one that keeps your rats happy and healthy.