Hmm, let's see, I think the 2hu game that makes me the hardest is probably IN. There's really a lot of lewd text in it! For instance, if you choose route 6A at the end of stage 5 as Border team, Reimu mentions that she and Yukari had been shooting off Reisen's clothes throughout their fight, and then she threatens to shoot the rest of them off. Ahh~ Also, Youmu criticizes Yuyuko for spreading her "shots" so wide. Youmu! Please! My goodness! And this is really only the tip of the ice berg. I don't want to spoil any more of the game for those of you who haven't played it yet, but, yeah, very lewd.
I think the 2hu that's easiest is probably HRtP, because most young boys probably pray to have sex at some point, so, well, okay I think you know where this goes.