Honestly depends on what you seek but i would recommand to start with either Demon's souls, Dark souls 1, Bloodborne or Sekiro :
-Demon's souls is the start of it all and got some brilliant ideas (the tower of latria experience), not too difficult especially with magic builds but maybe a bit clunky and the world tendency is confusing at first.
-Dark souls is the "first" souls game and it carries a big legacy. It's still clunky but it offers a good idea of why the serie became so popular : good difficulty, diverse areas and thrilling bosses. The dlc is really good too !
-Bloodborne is a true masterpiece : fluid combat, diverse weapons, some of the best bosses, areas and musics FromSoft ever did (the dlc is a masterpiece and it's on sales until christmas). Returning to older games like dark souls 1 or demon's souls might be harder after the "freedom" of movement offered by Bloodborne.
-Sekiro is a truly amazing game and a different take on the FromSoft formula. A total freedom of movement (you can JUMP !), a new combat system focusing on parrying instead of dodging, shinobi tools and stealth. The difficulty maybe too much at the start but once you understand the combat system everything will click together.
Dark souls 2 is a nice game but got major flaws (overall mediocre bosses, poor enemy placement and disastrous hitboxes, the dlcs are great tho) and Dark Souls 3 is, at least personnaly, an "enchanced" version of Dark souls 1 and as such a really good game but even better after experiencing the original.
Have fun and good luck on your journey !