Arguably, the least pay2win would be x-overd. Stamina recover in xoverd is super fast and girl can be easily collected through gacha. However gameplay is not as good as fkg and senpro and some girls with full job cap are sold more than 5000 nutaku gold (50$).
Senpro have more attractive deal than fkg and x-overd at the moment . 2500 nutaku gold for 5 draw of highest rarity girl (there is limited rarity girl but not yet come out) and 3000 for normal deal. However it's super difficult (nearly impossible) to get top rate girl through f2p. Moreover the game forces players to pay a small amount of $ to open auto-function.
FKG has a bit similar gameplay as Senpro's and offer more expensive deal 5000 nutaku gold (50$) for highest rarity girl. It have larger pool of girl and less girl dependent than Senpro (less diffculty) so you can still f2p. In my oppinion fkg > senpro mostly due to the design of girls and skill in battle.
I ve play Osawari island before, although it doesnt have best gameplay compare to 3 previous titles but it have best h-scene.