I'm not sure quite what you mean by "most secure." We don't have ratings comparing the safety of different seats. Correct use is a huge factor in the safety of a seat though, and honestly Diono seats are often difficult to use correctly and have installation issues in many vehicles. The 100 pound weight limit is also misleading because it is very rare that it can accommodate a child that weighs 100 pounds. As someone said above, the booster function is usually outgrown before the child is heavy enough to use the booster, and so most kids outgrow the seat the same time that they outgrow the harness. There are many convertible seats on the market that have a harness that will last as long (and in some cases longer) than the Diono harnesses.
All that to say - if longevity is a primary goal, then Diono is not terrible, but is definitely not as good as it looks on paper.