[2.5] Oro's Crit Flicker Assassin - 300k+ dps, 6k+ hp, Low to medium Budget, uber lab viable

Attack
Flicker Strike - Multistrike - Melee Splash - Increased Critical Strikes Support - Weapon Elemental Damage Support - Fire Penetration Support (maybe Crit multi support instead)

in a 5L drop the Fire penetration

Movement
Leap Slam - Faster Attacks Support - Fortify Support - Ancestral Warchief (alternatively you can use Power charge on crit as the last link: It's a good option to generate power charges in hexproof maps and against single bosses)

Heralds & Curse
Herald of Ice - Herald of Thunder - Curse on Hit Support - Assassin's Mark

It can be a problem to get enough Int in order to run Herald of Thunder - see passsive tree section.

Utility
Blood Rage - Arctic Armour - Ice Golem - Vaal Grace or Vaal Haste

Please Note that the gear shown here is what i use right now, but in most cases it's not necessary to have exactly these items - i will mention alternative options in each section.

Chestpiece

Kaom's is clearly Best-in-Slot, alternatively or as a stepping stone Belly of the Beast can be used. Other Uniques like Daresso's Defiance aren't bad, but with the current passive tree the build would end up having low-ish hp to be in melee range.

Weapon

Mandatory. For Leveling from 51 to 68, Terminus Est can be used - the damage won't be great, since Terminus is phys based and doesn't synergize well with our passive tree, but it's good enough.

Helm


Rare with Life and res. It's quite important to get Accuracy and Strength on a few pieces of gear, the Helm can help there. Please not that a WED master craft would be better than the Armour/Eva mod shown here.

Gloves

The awesome Southbound gloves! They come with a lot of life, res and ele damage; exactly what we want. What about the drawback you ask? We ignite everything that can be ignited and the burning damage is not a "Hit", so it will still kill enemies... and on top of that we do freeze & shatter most mobs anyway. Also this is the best among our items to get the socket colours needed for the Herald-Curse on hit setup (3b1g).
If you can get a Ele weakness on hit Corruption, that's great - if not: still use them.

[EDIT:] Essences bring new options to the table that weren't available in 2.3: You can now craft or buy Rare gloves with a crit mod. I would consider them almost as good as Southbounds - Look for something like: Life, Crit, Attack speed and/or Resistances and/or Accuracy.

If you can afford it, then Atziri's acuity is a great choice of course; the instant leech makes us a lot safer.

Boots

Windscreams are a good choice if you have the Curse on hit gloves and want to dual curse. The movespeed doesn't really matter because we have Flicker and leap Slam to get around.

If you don't need a second curse, use Darkray Vectors or rare boots... Darkray Vectors with an additonal Frenzy charge corruption would be a pretty strong option.

Belt


Weapon Ele damage, Life, Resistances, Strength... pretty obvious what to look for.

Jewellery


Above you see several examples of Jewellery i use(d) - the amulet needs to be upgraded though:) You'll want Life, Resistances, Weapon Elemental damage, Accuracy, Flat Ele damage (fire if possible). Getting some Intelligence here is important if you are going to use passive tree version 2 - see passive tree section.

[EDIT] Since 2.4, the new Opal rings and Crit explicit crafting essences are available. An Opal ring with WED, Crit, Life etc would mostl likely be better than the old Diamond rings.


One last thing on gear:
Do not underestimate Accuracy! It's important that you keep your chance to hit in the 90's - the higher, the better. If you wonder why, have a look at the wiki's entry on critical strikes: <<Accuracy measures the chance for an attack to hit. Accuracy-dependent attacks make a second accuracy check to confirm the critical strike. If the check fails only a normal hit is dealt. Thus accuracy can play an important role in quantity of critical strikes.>>

Version 1 "default" more int on tree = easier to gear (please note: in 2.3 i didn't feel it necessary to take Phase acro. For 2.4 i would recommend taking it.)
Version 2 "advanced": using a fireborn jewel below duelist and has less int on tree = harder to gear:

On Jewels you should mainly look for Melee Damage, any ind of Attack speed, Crit Multi/Crit Multi with Two Handed Melee Weapons, and Life.
Besides that, you'll probably want to get some Elemental Resistances, Strength and Intelligence there to make it easier to gear your Char.

Other good mods include:

Fire Damage
Crit chance in all variations: Global/Two Handed Crit chance/Melee Crit/Crit&Accuracy
Increased Accuracy
Chance to Ignite & Ignite Duration