I prefer a game where I can have some control over actions and events. An NTR game where the direction of the plot and its decisions are not obvious is quite difficult. For me the best ntr game would to have 5 characteristics.
1- Avoidable.
2- Repel competition or enemies.
3- Strengthen or weaken the MC's bond with his girlfriend with events and decisions (Not so obvious, of course).
4- Let MC's gf make decisions through values within the game.
5- Hidden, causing suspicion.
Example: MC's gf sees how his loved one has killed a lot of competitors or enemies without an apparent justification or explanation (Fear). The deaths have a justification, to prevent damage to the relationship, but MC's gf has her own values and convictions, which brings her disgust for the actions of her lover (Distrust). Certain people that MC killed were of importance to his girlfriend, romantic or not (Anger). Or it could also be decisions that disappoint MC's gf and that several of them will destroy the relationship.
Basically a game where you have to have a certain balance in your decisions or way of playing, where without realizing it you can trip over your own foot causing what you were preventing to happen because of your own fault.
In fact, there a 2 games that is very close.
Spiral Legend V [Expiacion] and
Corrupting My Girlfriend in Another World [R1leyD4rk] Also games from Kegani Laboratory, BBQ Lover, The Church of NTR and some games from Circle Meimitei.
Something that some mentioned games share is management? maybe? decision making and the use of time as a mechanism against us. But I feel that these games are missing something or do not meet the 5 characteristics. They are good games anyway.
But I think the closest thing to this would be real life lol. So let's move on to touch grass and establish interpersonal relationships with the people out there.
Perhaps the best NTR game is the one where you don't know it's there and when you get to a point in the game it surprises you making you tear your hair out due to some poorly made decision or the lack of some resource and poor management.