Best SK Race for Questing

[DeathsSilkyMist]

Troll if you mostly solo, Ogre if you prefer to group/raid.

Troll Regen on a Shadowknight specifically is going to give you way more bang for your buck when soloing over FSI. You will have a lot less downtime, and similar starting stats to an Ogre. You are going to be fear kiting a lot of the time, which means you aren't getting hit much, so FSI isn't doing a lot for you.

FSI becomes better in group/raid situations, because racial regeneration isn't very fast. You will probably have a healer, and they are going to be keeping you at safe HP levels, not your regeneration.

Iksar is generally the worst choice because you can't use Blood Ember clickies. Greenmist is not a very good weapon, and will get replaced by better ones. It's also not worth it to try and stack Epic + Greenmist. You will only get like 100 extra HP over simply procing Epic twice in a short timespan, and you will probably end up losing more HP due to losing DPS while swinging Greenmist.

EDIT: Quest-wise you can fix your faction on any race, so race isn't really a hard requirement, with the exception of Greenmist since it is Iksar only. So if doing the maximum number of quests possible is your primary goal, then maybe Iksar is the right choice. But you may regret it later if that goal changes.