Ren'Py is my favorite, no doubt. RPGM can be okay. It's a pretty big time sink usually, but depending on my mood I'm okay with that. Least favorite is Unity, but I suspect that may not be the engines fault. It just seems like a bad fit.
Ren'Py, due to the ease of access to story and lewd content. When people use RPGM, it often gets in the way of these two factors and introduces unneeded and unwanted elements.
Ren'Py is used by most indeed - and I think is a pretty solid engine.
Regardless of what engine is used though, in the end the developer and his skills for animation etc are what carries the game.
like other have said. RenPy is good. Its easy to use. It have features that are usefull (skipping, auto saving, keeping saves between game versions, and with the use of mods you can use cheats to skip pointless grind) But there is a but. If a game is poorly done, with not fitting menus, images not maching each other, it will make the game not look like a game at all, more like a something put togather fast in few hours. Not good at all. While even shit RPG maker game feel like a game, bad game but still game not a slide show. For this reason I can play bad RPG maker games longer then I can play bad RenPy games. But then again in many cases it would be better for a dev to just go RenPy and focus on content , rather then wasting time learning RPG maker scripting.
As almost everyone else, I say Ren'Py first, and a total no to RPGM.
I gve RPGM a real try, downloaded the two RPGM games that was rated highest on F95. After half an hour with each I gave up.
I have tried a few adult games in Unity and Unreal Engine, but didn't really like it. Yes, the moddability and hackability of Ren'Pyis a major factor for me, but more important is to be able to save anywhere and rolling back.