Hardest paladin oath to keep?

[Malaficus Shaikan]

I am interested in challaging myself.
I keep reading oaths are extreemly poorly designed.
What is the hardest oath to keep and why?

[Kuno]

I felt Devotion is the hardest to keep... Then again I never played Ancients. But I did feel like I had more playing room with Vengeance than Devotion.

[Jaggid Edje]

I have only played Vengeance, which is stupidly easy to keep. My guess, however, is that Devotion would be the hardest. Just from reading its tenants.

[Hobocop]

Probably Devotion because you can't lie or cheat, so that leaves a lot of options for talking your way out of encounters off the table, though you DO get extra Paladin options to talk your way out based on the sanctity of your word.

Ancients is easy peasy as long as you don't wantonly slaughter innocents, defile the natural order, or submit to questionable magical rituals.

[Chaotic Entropy]

Chastity.

[Dr. Xorin]

Agree with the other comments that Oath of Devotion is the most restrictive for sure. If you look over its tenets - You can't lie, you can't attack enemies you've parleyed with (ie. resolved encounter through a persuasion check) in order to surprise them, in some cases not during what is morally good/right (ie. helping the gnomes escape the forge in act 1) and attacking innocents.

Oath of the Ancients tenets allows for more devious actions, but attacking innocents is still a big no-no. Oath of Vengeance on the other hand is the most relentless of all the oaths and in my opinion the oath you are least likely to break during your campaign. I took the vengeance oath in my first playthrough and never had an instance of breaking it.

This video from MrGeeBee can also give you an idea on this topic: https://www.youtube.com/watch?v=AzljoOJ8sBU

Hope it helped