Depends on your goals and how committed you're to learn
If you want to make a simple porn game choose RenPy and call it a day. The simplest one to get straight to development
If you want to make more of an action-ish 2D game and willing to spend some time, depending on how fast you learn and how stupid you are then "Game Maker: Studio" is actually a nice option that is easy enough to start and opens a lot of possibilities
If you wanna make a career out of it go with Unity 100%, nothing more to add here. It will take the most time out of three but yield the most results, but is not worth it if you just wanna start making simple enough 2d porn games
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I do not recommend Unreal Engine under no circumstances, this one will be the hardest to learn and is pretty niche, used only for relatively big 3D projects, including AAA. And I didn't even see so many job applications for that compared to Unity
RPGM is just trash
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As for most popular for porn games it is definitely RenPy. Simple, approachable and suitable for porn games
Most popular overall is Unity