Noticed that none to Team Niimii's games have been recommended so allow me to fix that. For the record they are all RPGs with pregnancy as a mechanic and almost all feature some form of NTR.
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: Female PC. Fairly straightforward RPG which is to be expected since this was the circle's first game. Can be impregnated by both humans and monsters.
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: Female PC. Also a standard RPG but with a fun twist. The PC has a disease that will turn her into a tentacle monster in 100 days unless you finds a cure. However having sex with a man will delay the transformation and give you more time. It actually gives you a reason to go around having sex. And of course there are a plethora of different tentacle monsters to fight/get fucked by.
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: Small sequel/stand alone expansion of the first game. Once again can be impregnated by men and monsters, though there's only one human dude to have sex with.
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: Male PC, three female MCs. The first of the circle's games to heavily feature NTR. Player can romance and impregnate three girls. All of the girls can be NTRed, some sharing similar events while others having unique ones. AFAIK there doesn't seem to be any penalty for letting them get NTRed so feel free to do whatever.
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: Female PC. There are four different endings, one for a normal playthrough and three depending on which male MC you choose to be with (boyfriend with glasses, red hair dude, village chief). There are also a few minor NPCs to fuck as well as monsters that can rape you mid battle after you lose your virginity.
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: Male PC, five female MCs. Player can date and impregnate four of the MCs (the fifth is his little sister and thus off limits) as well as a few nameless female NPCs hidden around. Is honestly my least favorite of the circle's games due to how mean it is toward the PC if you allow any of the girls to be NTRed.
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: Male PC, two female MCs. This one is unique in that it's more of a light survival game where you need to go around gathering materials for various purposes. Can get very grindy.
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: Female PC. Another more feature rich sequel to Mii no Tabi. Largely the same gameplay-wise though sex scenes are now animated.