To sum up in my opinion, chaosmarauder is exactly right with all of his advice. When in combat against something with great armour saves such as space marines, or even terminators, powerklaws work WONDERS. They are worth all the 25 pts that they are. They many seem expensive, they may strike last in combat, but put wounds on your boyz to keep the nob alive, and he'll smash anything in front of him that isn't a land raider (and even it's 1/6 chance to glance or 1/3 and the possibility to PEN A LAND RAIDER on the charge). Another must-have that chaosmarauder pointed out is the bosspole. This will literally save your life (or at least your orks' ones) and in most cases keep them from running away, and keep them running towards your opponents gun line like they should be.
Deffkoptas, are the amongst the best harassment unit in the game of the 6 and a half years I've been playing, as they can start far out from your forces, go very fast, and blow up weak rear/side armour of your opponents prized tanks. Plus they have
tl rokkit launchas, meaning they'll mostly hit. (Mostly by ork standards). On top of all that, they're relatively cheap points-wise for what they do, and many times I've literally flown round the back of my freind's leman russ, blown it up, then cause havoc assaulting his backline heavy weapons. If there's one problem with them, it's their leadership. But with a little luck they'll either survive the shooting through jinking and decent toughness, or not fall back that far. All in all, you'll definitely want deffkoptas, I always bring at least one trio of them to every ork list I have. (Except for >300 point games

)
Trukks, these are relatively cheap, armed transports that are all but reliable. Don't take this the wrong way though! They can contest objectives relatively well if taken as a dedicated transport by boyz, come with a free big shoota that won't hit anything and can go very fast, taking up to twelve orks with it. Along with a near neccessary reinforced ram they're only 35 points, which is less than six orks, whilst going faster.

If you do shove some boyz into it, give them 'eavy armour or look for other, more armoured units to go in there. When the trukk gets shot, only one or two (if that!) will die, and even after that, they can carry on running the distance towards your opponent! Taking these is almost never a bad idea, and they're an unreliable way to get some green, crazy football hooligans with axes crumpin' some puny people!
Gretchin. I love these things! They're the most cheap infantry I think there is, along with conscripts with the imperial guard (which are less accurate, jeez) and only a couple of others. Although this does NOT redeem they're pathetic stats, they can literally fall flat on their face on objectives, keeping it capped for your team. Another fun thing to do, is bog down enemy units. If you assault a couple of long ranged elites, with little skill in close combat, you'll probably keep them there for a few turns, if not the whole game, if not YOU'
LL KILL SOMETHING WITH GRETCHIN.
Hope this helps you out with collecting your orks! And remember, boyz before toyz (as chaosmarauder and many others say)