These are some good questions, the "best" incest game dev is subjective though.
First you need to consider the "incest boom", which has infiltrated most facets of adult media, the volume of step-incest porn around for the last couple of years is no joke, in fact that came in around the time of the incest boom for adult games.
So what started the incest boom? good question, i would say that one of the big reasons(not the only one) is the fact that incest came back into the public arena with game of thrones, yep, as pop culture goes it's one of the biggest hitters in the last 15 years and along with the man bun, ash blonde hair and nudity in mainstream drama, incest got a serious boost due to GoT IMO.
Now considering the innate boost that GoT provided that lead to more searches for incest, therefore any existing incest games benefited, then consider that this coincided with the advent of Ren'py, a simple no frills VN oriented piece of free software most people could work reasonably well with a little practise and suddenly we have a new one man show in Lewdlab's Dreams of Desire go from a little pisspot game to one of the biggest games this site has ever had, of course the other notable game of that era was "Big Brother", which while slightly older and non-Ren'py did obviously benefit from the newness of the incest boom and in many ways was the sandbox counterpart to DoD's VN.
Next up we need to consider ICSTOR, who had been making games in RPGM for some time before the boom, not all incest but mainly, i think he saw a niche and went for it personally and fundamentally he's been making the same game over and over for some years now, each time quality rises but he knows his market and generally is a tried dev, far more so than many others at least, obviously he has benefited hugely from this combining of Ren'py and incest, his most recent work "Milf City" is his most popular (and probably most lucrative) game to date but i personally get the feeling that he'd still be making games, perhaps not incest games, even without the boom.
There are others that came latter, after the initial boom, that are worth talking about too, Timestamps, Man of the House, The Twist and Dating my Daughter are all important when considering the "incest game boom", some more than others IMO, DmD was a big deal but fell prey to patreon somewhat, MotH is almost a direct successor to BB, just better, Timestamps is mainly a VN and as such owes a lot to DoD and The Twist IMO is a pile of shite, but what do i know.
Which dev is most influential?, for me, Lewdlab, he was in at the beginning of the boom and had the added bonus of being both decent and being one of the pioneering adult Ren'py game devs, so he was on the right side of history twice over, could be luck, could be a level of savy that we don't normally see or a combination of both, either way he has a lot to answer for.
So, TLDR
The boom was created by a variety of factors:
The rise of incest in wider media, (GoT is a major culprit)
Ren'py coming onto the market and being easy-ish to use
Daz putting out the G3 era of models which both looked good and where relatively easy to use, providing decent CG for a manageable cost
IMO the ones at the start of the boom where most influential:
Lewdlab for DoD
Big Brother (i forget the dev and you should too)
ICSTOR, for being there before the boom, riding it and most likely being there after
Who is the best incest dev?, no idea, it's subjective.
