Shinobu Sensui and Suguru Geto are both characters in my top 3 villains list which makes sense due to the fact that they're both so similar, in fact Gege Akutami has admited to being inspired by Sensui when creating Geto. However, I find myself liking them for different reasons.

With Geto what I like most about him is his relationship to Gojo and his fall to evil. When we first see Geto (at least not Psuedo-Geto/Kenjaku) it's in JJK 0, we see him at possibly the height of evilness: he hates non-sorcerers to the point of calling them all "monkeys" therefore disregarding them all as uncivilized yet he benefits off their money, in fact he hates them so much that on Christmas Eve he starts the Night Parade of One Hundred Demons which results in the death and injuries of multiple civilians in Japan and despite not hating them he's willing to kill Yuta, Panda, and Inumaki to achieve his goal of getting Rika to succeed in the Night Parade of 100 Demons (keep in mind he considers Maki to be a "monkey") But we also see that he was close to Gojo and in a way is still friendly to him, he doesn't seem to consider him as an enemy per say just someone in the way of achieving his goal. Not to mention we do see that Geto isn't entirely evil and does care for sorcerers with his two "daughters"(forget their names right now)

Sure all the things I mentioned above are interesting and are good for villain but if Geto was only in JJK 0 I don't see him making my top 5 Villains list.

What makes me put Geto in my top 3 villains lost is seeing his fall in the Hidden Inventory Arc. We see at the start of this arc that Geto's ideals are a complete 180 of how they were in JJK 0, instead of hating them, he believes that non sorcerers need to be and should be protected by sorcerers. We see the peak of his and Gojos relationship as well. However, as the arc progresses we get to see his decline. When he and Gojo are tasked with protecting and escorting the star plasma vessel, Riko to Tengen we see how the care for making her last day on special while protecting her from everyone trying to kill her. It even gets to the point where Geto gives Riko the choice to continue living her life and she is about to accept the offer but then she's killed by Toji Fushiguro, a man with no cursed energy. What further hammers the nail is when Geto and Gojo are carrying Rikos dead body and the entire crowd reacts with applause.

It then time skips to a year later, Gojo is only getting stronger, Shoko is a healer, but Geto isn't progressing at all and he's clearly in a bad mental state, He goes around absorbing curses which from what we're told taste terrible. We see his conversation with Haibara and how his ideals match Getos at the start of the arc and how Geto wishes he could go back to thinking that way. His conversation with Yuki further causes him to believe all non-sorcerers have to be removed. Then there's the conversation at the KFC(?) with Gojo the every famous "Are you the strongest because you're Saturo Gojo or are tou Saturo Gojo because you're the strongest." line. This conversation clearly has a deep impact on Gojo and he doesn't want Geto to go down this path. The final thing that hammers the nail into the coffin is how his "daughters" (again forget theor names and ik they're not biologically related) were being treated in their village.

As for Sensui, while his fall to evil is certainly interesting that wasn't what made his my favorite anime villain of all time. What I personally find more interesting is his parallel to Yusuke, his inability to realize the grey between the black and white, his refusal to truly believe the fact that it's him Shinobu Sensui doing all of these bad things therefore making different personalities just to runaway from it, his searching for a meaningful death especially after having only 2 weeks left to live, how the "fake" Sensui Seven represents the 7 deadly sins, and how the "real" Sensui Seven represents the 7 deadly sins.

In short I love both of these characters especially as villains and think they're amazing written. I also want to note that whenever I referring to my "top villains" I meant anime villains, I personally like villains such as Bane, Two-Face, Darth Maul, and Darth Vader a bit more but both Sensui and Geto are in my top ten villains of all time list. If you want to know the other villain in my top 3 anime vilalins it's Cell, certainly not as well written as the other two but I think he has a cool concept and love how he is the embodiment of a perfectionist complex

I truly believe Yu Yu Hakusho should be an anime series watched by everyone(at least if you're a fan of anime) Everything from the power system, to the fights, to the characters are amazing in my opinion. The develop of Yusuke from a teenaged delinquent who believes he has nothing and no one to live for and fights for fun to a man who had realized his purpose and fights for his friends and family is what makes him not only my favorite anime character but my second favorite character of all time. The chemistry between each of the characters is also a highlight of the series and the dub has a great casting and dialogue.

TL;DR: Watch Yu Yu Hakusho