Issei Hyoudou (Highschool DxD)

Okay, the pervert harem king, so where are you and more importantly who are you with?

The Academy? Easiest choice and you could start your harem king dream right in the school and edit the rules of the school, the dress code for women, how women are to treat the boys (or just you specifically) what the teachers must wear, how the classes will go, the academy can be your playground and the first step to taking the town and anything else you want.

Humans? While not as gorgeous as some of the Supernatural women, they're still hot and you can always edit a power that makes it so you can turn the women into humans. Or maybe you're like Cao Cao and want Humans as the supreme entities on the planet, but you'll not waste good potential sex slaves and will tame any and all women you decide you want.

Fallen Angels? I mean what's not to like? All the powers of angels with the vices of Devils and plenty of sexy women literally looking like lust incarnate. You could change Kokabiel to not be a warmonger or you could swap him out for Gabriel as the fallen and get an easy access to her.

Angels? Why not? An edit and boom, no chance of falling from having sex and giving into lust and desires. Corrupt the angels into your personal harem while they still keep their holy powers. Of course, you could always become disenfranchised with how Heaven works and decide to tear it all down by fully corrupting them all into fallen with the women being entirely loyal to you, then you can move onto the Fallen and other factions to tame and claim. Plus, you could easily wipe out all the asshole sections and get the Fallen to serve you by restoring them to full fledged angels, for a price of course, and then move on the Devils.

Devils? It's an easy choice, but you can be a king instead of a pawn, you can change and alter all the players in the game by switching out people for others and making them your pieces instead of others. Maybe swap Kiba for Irina and Gasper for anyone from the manga or anime you watch. Maybe Grayfia is now your wife and maid bitch while Seraphell is your fiance and both of you have made Sona your obedient pet. Maybe take the original Leviathan family members as your personal servants as punishment/recompense for siding with the old faction.

Youkai? You got Yasaka immediately and you can age up Kunou to get two horny kitsunes, not counting any that are edited in both for your pleasure and to increase the Youkai's power.

Mages? Get access to the magic and contacts while establishing your group as the dominant players.

Hero Faction? Work beside or replace Cao Cao and get various members as your harem since you've got access to the Chaos Brigade: Jeanne, Morgan, Kuroka, and several others. Maybe be actual heroes and you only kill the assholes and criminals of the various factions.

One of the Pantheons? I mean you got mythologies all over, if you want the games to exist you could take over as Atreus from God of War and thus be connected to both the Norse and Greek Pantheons. Or maybe you want to build your own Pantheon and assimilate the others to grow your power and the goddesses and female warriors all live to serve and worship you? What better way to be a Harem King than being the King of a goddess Harem?

Chaos Brigade? Ophis has an adult form, you can make her curious about sex and then tame the void dragon and then using her you can get the various female members into your bed and serving you, maybe edit yourself so fucking them increases both of your powers. Of course, you could always just make Ophis a good little pet drago-slut for you and if the women want a power boost they need to serve you and the men need to provide females to serve you for a boost.

Somewhere else in the world? Why serve others when you can just make the world your own how you please and be anywhere you want as you do things. Maybe you make Themyscira from DC a real thing and the Amazons are your devoted harem of soldiers and slaves. Maybe you've got hidden areas of the world to explore like Harry Potter, the Sorcer Sanctums of Marvel, Hold outs of Elves and other creatures. Or maybe you just want to make the world into a world full of huge titted women that love to serve you?

Or Maybe even something else that I've yet to think of?

Regardless, there's plenty of fun to be had for a pervert and future Harem King.