For PvE, the short answer is none. If you are in a group, it's the healer's job to heal you. If you are solo or prefer not to exclusively rely on a healer, your heals should basically be Swallow Soul, Sap Essence, possibly Pale Order. Swallow Soul is a HOT. Some harder PvE content can be soloed by mixing Swallow Soul and Sap Essence at all times, even if your target type demands only one or the other for doing damage. Swallow Soul being a HOT keeps running and stacks with Sap Essence. As a PvPer at heart, but playing in PvE, I tend to also have Shrewd Offering in my builds, but strictly as an emergency heal. Some people prefer Harness Magicka over Shrewd / Healthy Offering. Shielding sets, notably Hexos Ward and Iceheart, both sets that also include damage via crit %, are further options.

In PvP, the short answer is you should use all of the above. The only issue is bar space. Resolving Vigor competes with Regeneration as a HOT. I would say Vigor is better, at least in solo play. You get the resistances. You get to activate part of Wretched Vitality, if you're using that. It can't accidentally go to another player. You're not forced into resto staff. Stamina sustain is something you have to account for, but since it's not a burst heal that you might spam, it's generally OK. If you're not using Swallow Soul, then having a dedicated HOT skill, like Resolving Vigor, in your build is practically a must. I'm not sure how good of a heal Refreshing Path is, as I don't use it. Enemy players can see it. While it's hard to find the player who left that on the ground, it always struck me as a daft skill for a cloaking nightblade. It's more for tanky brawler nightblades that use Dark Cloak. I could be wrong.

HOTs are proactive skills while burst heals are reactive / emergency skills. You generally need both in PvP. There's not much difference between Healthy and Shrewd Offering. I would say that, if you're generally stacking heals and are dodge rolling damage, Healthy Offering should be better. I, however, use Shrewd Offering as I run a 1H+Shield back bar. My build is extremely fast, basically at or near the speed cap at all times. I do not use Shadow Image. Some people prefer a resto back bar historically or nowadays perhaps a bow. I, however, find there are occasions where your only choice is to block. You're caught out, you have no time to cast Race Against Time, you have no easy line of sight to dodge roll to, perhaps you have a Meteor incoming or you have the detection orb on you. In those cases I tend to block spam Shrewd Offering. I choose that morph for the lower cost and the lower stacking penalty.

You have to pay attention to every little thing in a PvP build. My tankiest setup includes 1H+Shield, Swallow Soul, Siphoning Attacks, Magic Damage / Restore Health Enchant, Shrewd Offering, Resolving Vigor, Reviving Barrier, Mara's Balm and Rallying Cry. That's my low damage / bring Tel Var home setup. Light armor, but feels tanky indeed. You can strike a different balance, but in any case it's worth being conscious of all your healing factors and how they add up. Swallow Soul may seem like a small heal in PvP, but all that small fry does add up. I find the long-lasting DOTs in the current game can take a toll you want to offset. Same goes for Siphoning Attacks and the other factors. It's harder to make them work, as some of the heals only proc while landing light attacks, but what else is new for nightblade. They certainly help in 1v1 situations whereas, if you're being focused and forced into defense, you're going to lean on Vigor, Shrewd Offering and Mara.