Ren'py is honestly the most popular and easiest to use so I'd have to go with that and especially since AVN's are mostly Ren'py as well which is my favorite genre.
Unity would be my first choice but it seems that with only one guy working on the game, it takes a lot of work to only make a smell dent into the mountain before them. Renpy is probably easier, at least it's easier to play with.
RenPy and RPGM but it really depends on the type of game that is made is made for each engine, i've seen some Visual novels made in RPGM and most if not all of them suck, and Ren'py has more flexibility but i still don't understand why people try put any kind of combat in RenPy game it just don't work well
I think Renpy is great. Although I wish devs would have better animation when they choose to use it. I know it's a lot of work, and I'm not familar with Renpy enough to know if it's up to the task. I've just seen more than a couple games where there were great visuals but then have janky half second animations.
I have a dumb question, does renpy allow animation when choices come up? For example, instead of a still image, can the character have an idle animation?