Me a month ago:
"Eh, I'm kinda burned out on the Rance series. Collapse of Zeth was better than expected, and the older titles are kinda neat. But Sengoku Rance was not that great in hindsight. I probably won't have any strong feelings about Rance Quest now that they've been stalling it for so long - I was apathetic when they finally,
finally finished the translation of Fate/hollow ataraxia too. I guess I'll give Kichikuou Rance another playthrough, but that's it."
Me right now:
"HOLY FUCK IT'S HERE, AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHH"
Anyone can tell me which Rance game I should start with?
That's a pretty good question.
The setting is *huge*, with hundreds of characters and 20+ games. Not all of them Rance games, either (the Toushin Toshi series, Kaeru nyo Panyon, Evenicle, etc.).
But, the Rance games all tend to be pretty accessible even if you haven't played any of the other games in the series. Yes, there will be a lot of references to story events in earlier games, but you won't
need to have played the other games in order to get the plot. Knowing the stories of the other games is a bonus - but each game has a self-contained plot.
Another thing to note: the Rance series had a pretty big makeover between Kichikuou Rance and Collapse of Zeth. Both the art style and the writing changed. Collapse of Zeth is the first of the really 'modern' Rance titles, though Sengoku Rance is the most well-known title of the series (in the west, at least). The other 'modern' Rance titles include the ones set after Sengoku Rance (The Helman Revolution and Rance X, which comes in two parts), as well as remakes of the first few Rance titles (up to Rance III, IIRC).
So, if you want to pick a Rance game to be your introduction to this series, then it's best to start with one of the 'modern' games. I would recommend Sengoku Rance. The remakes of the first three Rance games might also be worth it, but Sengoku Rance is kind of self-contained while also having just enough references to the wider setting to make you curious about the other games.
You
could also start with the first few games in the series, but those are pretty primitive by today's standards. And more importantly, the tone and writing in those games is different. I actually like the older games better, and I still think that Kichikuou Rance is by far the best game in the series - but the newer Rance games are very different overall. So like I said, it's probably best to start with one of the 'modern' games.