I mostly agree with ZeoW about stat priority with one exception: I consider METER GAIN being more important than HP for defensive fighter and on par with ATK and PIERCE for offensive one.
High METER GAIN for a defender means they can charge their BB3 quicker and land a devastating blow to the opponent - that's why Meter Control is so important in Rifts and high PF streaks. Offensive fighter with high METER GAIN can charge BB1 and BB2 in one combo string, significantly increasing the resulting DPS.
Speaking about Silent Kill (and Val in general), you should aim for having at least 45% METER GAIN. This will allow you to fully charge Countervenoms in one combo (L5->Launcher->Juggle) and land at least two of them right after the juggle (CV->D1->CV, you can finish with Forbidden Procedure immediately after second CV). CVs are your main damage dealers, so the ability to spam them in every combo you land is too good to miss. When you get there, you can continue focusing on ATK and PIERCE. CRIT RATE is not that important IMO unless you use a fighter that relies on crits heavily (Claw and Order, Parasite Weave, Double with Chaos MA...).
Wow, Thank You Disasterrr19 I will take this into consideration as well, I want to Diamond my Silent Kill first out of all my Gold Vals first, for me she's my best Val and next to my BBath Eliza my best fighter So far
I also agree with ZeoW. I would just like to add something. Valentine as a fighter has a range of health between low-medium. So while she isn't as easy to kill as Peacock, or Ms. Fortune, she can still die faster than Big Band or even Eliza. So likewise, try to invest with BBs that have +health stats.
Also, Disasterr19 is right. The more you can use your BBs, the quicker you can end the fight. Val's BBs can pack quite a punch, so as much as possible, try to upgrade it as much as you can.