Ves considered the Valkyrie Prime to be the Valkyrie variant with the highest potential.
In fact, if he had more confidence in his latest creation, he would have gone a step further and declared it to be his strongest Valkyrie mech straight away!
The reason why he withheld this judgement was because he couldn't definitively state that the Valkyrie Brunhild performed worse than the Valkyrie Prime.
At worst, the Valkyrie Prime sat between the Valkyrie Avenger and the Valkyrie Brunhild. The lack of certain key strategic materials prevented Ves from replicating all of the advantages of the latter.
Ves still felt sore about his inability to obtain Rorach's Bone. If he could add even a modest portion to the Valkyrie Prime, the mech would definitely be able to synergize well with Joshua's force of will!
As it was, Ves and Venerable Joshua had to make do with a mech without any inherent regenerating properties.