What melee weapons are the best? and why does the Kett melle weapon that adds fire damage not prime

As Unicephalon said: The cryo gauntlet is the best, despite is moderately low damage.
It hits fast, instantly freezes unprotected enemies, and sets up Cryo explosions that can kill, hurt, and/or immobilize entire groups.
And it receives bonuses from Cryo Beam and Snap Freeze.

The kett fire swords takes two (or three, can't remember) strikes in a relatively short timeframe in order to prime. Or someone shooting the same target with incendiary ammo for a bit.
The electric firaan is even worse, and takes one or two strikes more to prime on its own.
(Though there is a fix for that somewhere in this Changes Pack)