Rance 4 is my biased opinion coz I can't stand the old UI.
Did you try TADA's patch?
I would recommend Rance 1 (obviously), Rance 6, and Rance 7 to start the Rance series
Rance 2 introduces Maria, Shizuka, Mill, Milli, Elenoa, Bird, and Feliss. It's also not that long a game. Really I wouldn't skip any of the first three games because none of them are a huge time investment and their difficulty curves aren't going to have you bricking entire runs. And that's without the obvious, "This is where we first see most of the franchise's core characters" thing.
Rance 4 is semi-skippable. I'd definitely try using the TADA quality of life patch before swearing it off entirely, though. It's the first appearance of Athena 2.0, and Freak. It also continues a B-plot from the Patton arc from the previous game. And it's also the first time Alicesoft sits down and starts fleshing out the world lore for things like the Holy Sect and the Warlords. Rance 4 is where the franchise goes from goofy porn games with gameplay and plot to an actual world with jokes and porn and gameplay.
Rance 5 is
very skippable, because it only introduces two new characters and mostly serves to move Rance from the 4th game to the sixth game. Although Rizna's a great character and her story arc is somewhat relevant to the plot of Rance 6. And Compandon is just fun. To it's credit, even though Rance 5's gameplay loop and combat systems are weird, the game is also the shortest in the franchise.
Rance 6 I would discourage skipping. While it is Rance at it's hardest (pun intended), it's also the start of the (loose) starting point for the end of the franchise. What sets 6 apart from 7 is that where 7 has a small cast of call back characters, Rance 6 is absolutely stuffed with them, and is following up on B and C plots from previous games. About half of the roster of characters are from previous games in contrast to SR where it's more like 6 in a total roster of ~80. Rance 6 is continuing the Patton Arc. Rance 7 has Rance eating dango.
I've already covered Sengoku Rance but if you had to play exactly one Rance title, it'd be 7.
8, 9, 10, don't play without playing the rest of the franchise first.
Kichikuo can safely be played after playing 1-4, but it's a non-canonical spin off so the only pin I'd stick in it is to play it BEFORE playing Sengoku Rance for the same reason it's hard to go from something like Halo to the old PS1 / N64 era first person shooters.