They all challenge you on different ways.

DS3 is actually quite interesting to finish on SL1. You can cast one basic spell, lots of weapons are available to you, but you need to think carefully about your loadout so you’ve got a good set of resistance and buffs for each area.

DS1 limits you to terrible weapons with boring movesets, and by the time you’ve heard the pyromancy noise for the ten thousandth time, you’ll be ready to chuck your monitor out the window.

DS2 is hellish. Your dodge is tied to your agility, which only a few items boost, and for the first 3/4 of the game, you’ll only get that from a crap armour set. Only a couple of weapons are available to you. You’ll need to know basically every boss inside out; some of the ones that were comedic before are no longer so funny.

Scholar is like DS2, but you can’t run into the DLC early on any more to get some of the powerful rings that are in there. I almost cried when I finally got the ring of the embedded.

Wouldn’t know about finishing ER on RL1/no summons. Cleared out the four demigods easily enough, but Morgott was a run ender. RL1 does not leave you a lot of room for mistakes, put it like that.