Outlaw, generally. Mostly because of reaping.
Assassination is better for single target, and will usually pull more overall on tyrannical weeks because of better single target damage on long boss fights.
Assassination can also pull pretty freakin’ impressive AoE with the right azerite traits, whereas outlaw has no such trait option to pull pretty impressive single target damage.
TLDR, assassination and outlaw are close enough that you should play whichever you enjoy more. Outlaw is more popular right now because of reaping.
Edit: Though despite the popularity of outlaw right now, some folks still want assassination for some dungeons on tyrannical weeks. I was rejected from a 16 Underrot yesterday after being asked what spec I was.