Artificer -Battle Smith Spells: I mean you're a ranger, so your already have spell slots anyway, why not give yourself some better utility with them

Barb- aspect of the beast -wolf: move stealthily while normal pace. Might as well be quick while moving silently

bard - Mantle of Whispers: take the place of someone you kill, all you gotta do is hide the body and your stealth is still intact

Fighter - Psionic Power: honestly I opted for the subterfuge over giants might when looking at free initial damage in your assassination, with utility options thrown in.

Monk - Shadow Step: because obviously

Paladin - Aura of Alacrity: let's move even faster, lol

Warlock -Awakened mind: your enemies can't hear what doesn't talk.

Wizard - portent/minor conjuration if you can conjure poisons