Omg do I really have to choose just one? 😫😫😫😫💕💕💕
I think it would have to be brocon! Always whenever any media has brothers I'm like "oh yeah they r fuckin!!!" The dynamic is very dependent on age and agegaps and stuff as well, but I think my main fav would be where they both enjoy it :) like no dubcon/noncon elements, purely brotherly love!
One of the brothers has to be more pushy with it usually, I mean how would anything happen if he wasn't pushy? Hehe
And the other brother is like "hmmmmm idk about this one chief...what if parents find out yadda yadda"
But eventually they get so absorbed into it they stop caring about anything anymore 😋 All that matters is that they'll always have each other when they're trying to get off! hehehehe

Throw in some elements of other characters being suspicious on how close they are, some guilt about doing it so much, and ~viola~ I'll be hyperfixated on them for 3 months minimum :3👍