Hello! Always goof to find a new yoof wanting to join da WAAAGH!
You've got quite the diverse set of foes there, so I'm not sure if you've perused it yet but the first page's post gives a good summary of the Ork units for 7th ed:
http://www.dakkadakka.com/dakkaforum/posts/list/643980.page
Besides that the general "must-haves" for Orks in most lists are:
Tankbustas - Usually taken in small groups of 6-8 in trukk dedicated transports, they give us the ranged anti armour we need on the go with decent reliability thanks to tank hunters. Also they are able to shoot up
MEQ and
MC units in a pinch.
Mek Gunz - Lobbas and Kustom Mega Kannonz are the go to guns of the Ork artillery. Lobbas are good consistent anti-infantry that doesn't require
LoS and barrage makes it good for sniping out squad leaders and characters. Kustom Mega Kannonz gives us answers to 2+ save units at range and are good at melting tough models, particularly Necron infantry since you force a -1 modifier on their
RP and ignore their armour. Also does well against Eldar Jetbikes. T7 makes these units surprisingly resilient. Ammo runts are mandatory, especially for
KMK Get's Hot! rolls.
Meganobz - A true Ork Bully unit, they are best used in two ways: as a
MANZ missile or taken as part of the Bullyboyz formation. A
MANZ missile is a minimum squad of 3 Meganobz with bosspole in a trukk that is fired off like a "fire and forget" missile at a target enemy unit that they can dominate. They are cheap enough to be expendable and dangerous enough that if your opponent doesn't kill it that they can wreak havoc on their backfield and vulnerable units.
The Bullyboyz formation comes from WAAAAGH! Ghazghkull supplement and deals with the Meganobz' leadership issues by giving them fearless, WS5 and Fear. Dead killy, but expensive since you need 3 units of 5. Meant for larger games.
In both cases, avoid going against the enemy elite, your ideal targets are those who can't get through 2+ saves.
Lucky Stikk Warbosses in Mega Armour - A true staple of the Ork army at this point, it is our codex's super combo, where in conjunction of being in a decent sized number of boyz supported by Painboy the Warboss can tank a ridiculous amount of damage if set up out front. Basically re-rolls all failed 2+ save rolls and
LoS the attacks that do ignore his armour onto his Ork boy bubblewrap. Requires a transport, ideally a battlewagon.
You can also combo the Lucky Stikk on a Biker Warboss as well, though you can't be quite as gung ho about attacks since there are a significant amount of ignores cover AP4 weapons nowadays.
Grotz -
El cheapo objective holders and pivotal for easy access to objective secured and unlocking Combined Arms Detachments to gain more access to heavy support and fast attack slots. Don't underestimate their ability to lift far above what they cost and if your opponent kills them, it means he's wasting his time on them instead of more deadly units.
Deffkoptas - Ideally taken as single man units, they are awesome for objective grabbing late game and getting line breaker. Outflanking also gives them good angles for potshots at enemy armour at the rear so it's almost always best to give them twin-linked rokkits. Other roles include having them soak up overwatch for your other units.
Warbikers - One of the more competitively priced units in the Ork book, they're cost effective, survivable (especially with a Painboy) and put out a lot of dakka. They don't have the weight of attacks most ork units have so it's best to whittle down the enemy before charging in with warbikers.