What is the optimal Melee build for Zero?

Optimizing Zer0's talents for melee damage is somewhat trivial. Just greedily spend points on talents based on the amount of damage boost.

Killing Bl0w 5/5  (500% melee against low health)
Ir0n Hand 5/5 (15% melee)
Be Like Water 5/5 (30% melee)
Backstab 5/5  (40% melee from behind)
Execute (deception override)
Foll0wthr0ough 5/5 (40% melee after kill)
Many Must Fall (deception modifier)

C0unter Strike 5/5 250% counter damage
Ambush 5/5 (20% damage from behind)
Death Mark 1/1 (20% damage against targets previously hit by melee)
Rising sh0t (45% melee damage when fully stacked)

Headsh0t 2/5 (8% critical damage - would be 20% crit at 5/5)

Not taken : it was the smallest bonus and I'm out of points.

Like the Wind 5/5  (15% melee while moving)

I haven't actually played this, so... here's some advice or things to watch for if you try it:

If you find you can't stack Rising Sh0t, move those points to Headsh0t and Like the Wind.

If you find you can't get behind enemies (solo play might be tough to do this), move the points out of Ambush and Backstab.

If you find out that your melee damage is ok, but you need more Deception, or survivability... go for those talents that support that. Start by removing points from Ir0n hand (smallest bonus on the list).

If you find that you crit all the time, move points out of Ir0n hand into Headsh0t.