As much as I love Ben 10 I cant deny that it can sometimes suffer from simplistic writing, especially in it's villain department.
Like, it's clear why Kevin was the fan favorite villain ; he's developed and has more traits than just evil, he is an in depth and interesting character. Vilgax is cool but what traits does he have other than sadism? He is evil to everyone he meets and negotiates with nobody, because he is one track minded and all he can think is "I want my Omnitrix and I want it now so I can rule the galaxy". Who is Vilgax besides evil? I won't deny, being willing to dissect children slowly or being a world-conquering war lord who is very okay with causing genocide on Tetrax's home planet (with Tetrax's unwitting assistance and subsequent abject shock and fear) is fucked up and I love that, but my god I wish his traits were developed more.
Even though I'm also a huge Dr. Animo fan, he hasn't many traits beyond being an entitled terror upon society and animal lover, but I enjoy his progression of being power hungry and desperate for more ever since he gained any power from a fraction of the Omnitrix, whereas Gaxy boy over here never progresses : send in the drones, get the assassins, beat up the 10 year old and get fucked up after saying "GIVE ME THE FUCKING ALIEN ROLEX TENNYSON I AM SO SICK OF ASKING" 78 times. The Big Tick did more of a number on earth than Vilgax ever did, and that thing lasted one episode and was destroyed by Cannonbolt alone.
The Forever Knights did just about jack shit too, as it always felt like they were stopped before their plans even began. Ben would stop them at step one most of the time, save for when they got to the sword past the God in that ancient temple, just for their plan to fall apart (literally) as the sword crumbled. Their biggest time as a threat was when they put ben into a dream to get the omnitrix, for once their plan actually progressed and they posed a true threat.
Even then, the Forever King shows up and does literally nothing after hiring every other human villain which only subsequently proved that Animo is the most effective villain of them all. The Forever King made everyone do the work for him, picked up some power thing, did almost nothing with it and was destroyed by Upchuck.
I wish more Ben 10 villains had the depth Kevin had, as Kevin is a phenomenal villain, and is only proven moreso in Ken 10 where he is a fucking despicable person to his son and has only sunken further into his maniacal nature and urge for superiority over others rather than seeking self acceptance and forgiveness to himself, leading a side-villain to he more in-depth and interesting than the main bad guy.