Okay I've done (a very small amount of) some research and everyone, for the most part seems to agree that the spear is historically the most effective melee combat weapon. But like, what about halberds? they had the same pokey abilities but with the added options to chop people or do a swipey pokey with this part of the weapon.
I'd recommend the reply from /u/WARitter here. It doesn't directly answer your question, but it addresses both the spear and halberd, including historically when one became favored over the other.
More generally, when you're looking at a superlative like "most effective in history," there are a lot of factors that you can take into account:
I can't answer all these points for you, I'm just trying to give another way to look at this. "Most effective" could mean, in a 1-on-1 fight, which weapon is more likely to succeed. But that's arguably a less useful question than "which weapon had a greater impact on history, and in that sense was more effective?"
A sub question: Isn't a large part of a spear'a effectiveness the relative ease in mass manufacturing spears? Tons of cheap spears means more people can have on in hand for training purposes, or am I off?