most effective melee weapon?

[Dux Corvanus]

Hahaha, I have one I like to swing around in the backyard and beat up trees with

Oh, I see...

[Zero1]

Hahaha, sometimes I also like to pretend I'm a viking berserker and the trees are monks at a monestary I'm raiding

[QwertyMIDX]

The advantages of the spear are:
(1) Stand-off ability - perhaps particularly useful against cavalry.
(2) Reach.
(3) Penetration (when thrust with two hands).
(4) Probably requires less training to be effective than some other melee weapons.
(5) Can be effectively used in relatively dense formations.

You forgot the biggest advantage of the spear, it's dirt cheap to produce.