It's situational for me.
If you want to be quiet and remain undetected, then the best thing to do is to either use your forearm or something else and choke them, crushing their throat. Once they become deprived of enough oxygen, then they're dead.
But if you're fleeing, in combat, or something else where stealth is moot, then a knife across the throat is good enough. You're likely to cut the jugular, and with the windpipe exposed, they'll likely panic, which would send even more blood into their lungs, and possibly drown them in their own fluids.
Come to think of it, human's main weakness is our necks..... we need to protect them!