Orks are not competitive.
Let's let that sink in for a bit. Really, they aren't.
I love my Orks, they're my favorite army. When Dawn of War came out I was hooked on those crazy guys. New book dropped and I was in whole hog, buying upwards of 2000 points of them before I knew what was going on. They stomped face in 4th edition, with its lean towards static gunlines and weaksauce vehicles. With every enemy I faced, the horde ran right over them.
Then 5th was released, and the floor dropped out from under us. Now my Orks are a "for fun" army like my Deathwing.
Orks aren't competitive in 5th edition the way Space Marines, Imperial Guard, Eldar, Dark Eldar, and Tau are. Let's examine this before people start flying off the handle.
What makes an army competitive?
A competitive army is one that can win against other competitive armies in a competitive setting, typically tournies. Simple, right? You have to be able to effectively take on anything without sacrificing your own ability to win. That goes from destroying vehicles to handling the infantry inside said vehicles or hordes or anything else you might see thrown at you. You must have redundancy, so losing 1 unit to bad rolling doesn't hamstring your army. You must have flexibility, so you don't end up relying on a handful of units to perform important tasks and see them made into priority targets against some foes and worthless against others. You must be able to win the game, duh.
Orks really don't do all of that.
Orks have difficulty taking out vehicles. Their only weapons to knock out light armor (10-12) at range are Deffguns, Kannons and Rokkits. Zzap Guns and the Shokk Attack Gun are too unreliable. Boomguns, Kill Kannons, Grotzookas, Big Shootas, and Wyrdboy psyker powers can work as well, but not nearly as reliably or require you to significantly alter your army composition. I didn't mention PKs because, let's be serious, they suck at it (walkers being the obvious exception). Chasing after a moving vehicle isn't going to net you good results. Even a charging Biker Boss, so 5 S10 attacks, will get roughly .28 Destroy results on a Rhino moving 12", .83 on one moving 6". Puh-leeze. If you're army can't reliably take out vehicles, you will struggle against any mech army, and considering all the mech coming out of the woodwork with 5th edition buffing them so.
Orks turn out pretty well for redundancy. You can field multiple units of Lootaz, Kanz, Buggies, and Boyz. However, all the good units compete for the same FOC slots. You can't have it all though, so something ends up lacking. If you want to have redundant units of Lootas, you won't have Tankbustas or Nobs, so you end up centering your army on the Lootas, or at least making them a critically important units. If you want those Nobs, you have to take a Warboss to make them Troops, giving you an obvious lynchpin unit. If you bring Kanz, you'll need more than 1 unit to have a reasonable chance of survival, so you don't get Battlewagons and can't field Nobs, Boyz or Tankbustas. Having all the good stuff in direct competition means your redundancy shapes your army around these units. While Guard can take tank busters and infantry killers everywhere and Marines get lots of reliable armor and heavy weapons across the board, Orky goodness is concentrated into Elites and Heavy Support. That sure sounds like reliance on a handful of units to do the job (destroying armor).
Orks only have a handful of really good units. Lootas, Nobs, Boyz, Buggies, Koptaz, Big Gunz, Kanz, and Battlewagons. Everything else is situational, expensive, or just plain bad. The problem with building an army around these is the aforementioned competition, you've got everything competing for the same slots.
Orks are one of the "noobcrusher" armies; they excel at stomping all over poor lists and players. They have a very shallow learning curve and can bring a lot of power to bear, but once you get familiar with how the game works, they plateau early and quickly. Vehicles in any appreciable amounts give them fits, Land Raiders in particular. Orks fold to dedicated assault troops getting the drop on them thanks to their low initiative, reliance on furious charge and No Retreat! Their uber units like Nobs have obvious and plentiful counters (S8+ weaponry, particularly ordnance, vehicles) and are damn expensive.
Orks are currently considered competitive because of this. They look good on paper and perform well early on, much like any new release. They are extremely punishing against armies not optimized for 5th edition. Sad to say, people still haven't caught on to the changes and keep running the same armies, so they keep on winning, feeding the illusion that they're hot shit. Despite its many flaws, the latest 'Ard Boyz shows this fairly well. The winning IG list was far from the best, but it was built with 5th in mind and unceremoniously crushed the typical Ork "powerbuild" of 180 boyz plus bitz. Once people figure out how 5th edition works (which I intend to facilitate as best I can), Orks will drop off the map like they should have a year ago.
The current crop of "competitive" builds for Orks really aren't all that hot.
1) Nob Bikers. Great a couple months ago, until people figured out how to beat them. Apparently sinking all your points into 1-2 uber units that can be killed off by battle cannons and can't destroy vehicles is a bad idea. Who knew?
2) Battlewagon Brigade. Tons of AV14 vehicles full of big fun guys. At first massed AV14 sounds good, but then you have the awesome Battlewagon model with huge side arc, open-topped AV12, and the same problem of handling vehicles. Also sinking points into big scary units of T4 dudes.
3) Green Tide. Because a ton of infantry with no chance of harming vehicles running screaming across the board is such a good idea, right?
4) Speed Freeks. Loads of Bikes, Trukks, and fast stuff. Pro Tip: don't put your faith in open-topped AV10 vehicles and 12 boyz that get killed or run off way too easily.
5) Clusterfuck. A little bit of everything, a whole lot of nothing. You know the type, 60 boyz, 1 Battlewagon, a handful of Trukks, Snikrot and Zaggstrukk. No redundancy, poor unit and weapon choices, and far too easy to pick apart.
6) Kanwall. A variant of the Green Tide featuring 9 Kanz screening your horde. Kanz are great, but putting everything into 9 squadron'd AV11 vehicles will end badly, especially since your army is so slow.
7) Lotsa Dakka. You don't see this often, but shooty Orks can be quite nasty. Lootas, Big Gunz, Shoota Boyz, and Buggies/Koptaz with SAGs and Grots for good measure. They still have no answer to heavy armor and are quite easy to beat in combat or rout.
That's not to say that Orks are completely inept in competitive play, they just have a much harder time than truly competitive armies. I've used my Battlewagon Brigade and Lotsa Dakka armies to a good deal of success in the past and they're still a blast to pull out for local tournies or casual play. But when the chips are down, I don't put my money on the Orks. The usual caveats apply as always, player skill > list, scenarios can screw up balancing by favoring some armies and nerfing others and luck always trumps all.