What do you think the perfect villain would be? Somebody who's awesome at gameplay yet can still be beaten. Someone who you can like while at the same time want to stop.

Ultimate villain for fallout would be somebody who can go all out to beat you ( like a CPU player) which does not need a huge robot scorpion or bad guys , he just does the same as you , use vats and use nukes

The perfect villain for me is someone recurring throughout the plot in different boss fights. It's not someone behind the scenes, but a regular confrontation to impede your progress. It doesn't even have to be bad intentions, just an antagonist to your story arch.

As @lukaboot mentioned it is even better if their strengths are on par with your's. It encourages increasing your skill, and not looking for a very specific play style in order to defeat the boss per encounter.

@lukaboot @beardofsocrates just as i was thinking a counterpart on par with your players combat skill.

Every characteristic of Ulyssess is great for me, he has a point in his objectives and not just a dude to who want be evil. Although he is fairly easy to defeat if you high lvl gear but somethin like legate lanius type of combatant would contribute to a great villain

The best villains are the ones you don't suspect.
Take a certain Bioshock One antagonist for example.

The perfect villain should be someone you don't suspect, someone who's not the villain because "i'm evil and i like hurting other people" but because he thinks he's doing the right thing, has a real reason for being the villain. Take Wallace Breen as an example, he was a VERY good antagonist

Perfect villan ? Wut ?I would go for the Chaos Gods .They are neither benevolend or malevolent.They are a collective subconsciousness of thoughts ,feelings ,dreams ,nightmares and other stuff related to the mortal psyche .So to defeat them you must ...purge the entire universe of all feelings .More or less.
Unfortunately for fallout series ,there is not villans .Those people are not evil because they like doing those things .For example Caesar does not consider him evil because he is conquering and burning half of the Mojave .Super mutant does not consider Him/Her/it evil because HE LOIKS TO SMASH LIL HUMIES .Its basically fight for survival in this brave new world . So the ultimate evil of fallout series should be the ...Ug-Qualtoth ...The Blackmanors and the Dunwich horrors .This manifestations are aware of what are they doing and take pleasure from it .