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When it comes to the greenskins artillery what would be most worth going for.
Doom diver catapult, Spear chucka or Rock lobber
I am trying to decide wich one to use

Why not all of them! Their warmachines are probably their best use of specials and rares since other than some misfires, they're relatively stable. Great for variety of jobs. I would at least have 2 Chukkas and a lobba or doom dive but I would rather have both points permitting!

Doom divers are ruddy awesome, they kill so many enemy knights and heavy armored units! I always field 2.

Rock Lobbers are ok, I guess. I only have one though.

Spear Chukkas are nice and cheap, but mine never seem to hit crap. I field 6 as often as I can, but I'm lucky to get 2 hits out of the 6 at long range.

I just looked up the doom diver rules again, and ya, they are pretty awesome for taking out armour.
I still prefer the rock lobbers though (my favorite weapon for any army that can get it): your biggest threats are going to be enemy hordes and units with enough ranks to take out steadfast. Chuck a few stones across the table each turn and even your (relatively) expensive orcs should be stubborn when they hit combat.
I like the idea of spear chuckas and have seem them do some pretty mean things, when they hit. Problem is you will have limited sightlines when playing greenskins (meaning less targets and more cover penalties). The ability of the rocklobba/doom diver to ignore these penalties is too good to give up.

New to greenskins, not sued to artillary my other army is Ogres so Rock Lobbas and Doom Divers see over your own goblins?

So you can set up an army like this: (facing north to south, opponents in front of the goblins)
X=Spear Chukkas
O=Doom Divers and Rock Lobbas
T=Goblins

___TTTTT
___TTTTT
___TTTTT
XXXOOOOOXXX

And not have any penalties if shooting straight? had to put in __ but think of it as open space.

I played against a list that had 6 spear chukkas. Scared the bejesus outta me. He fired every turn, and only 1 bolt hit the whole game.

Doom divers never hit a thing either. So I'd say the biggest thing with them is their inability to hit stuff. But they do seem fun.

It's true LoS. I'd imagine that, normally, a Goblin War Machine cannot see overtop other Goblins unless shooting at taller-than-Goblin enemies. Add an Orc Slaver, you get a LoS boost from a taller model.

Doom Divers and Bolt Throwers, overall, are the best artillery. Statistically, Doom Divers will land dead-on about 38-39% of the time. This is about a 10% accuracy increase over typical Stone Throwers. If you place in the center of a unit, and there's only about 2" on all sides of the dead-center, you'll hit about 50% of the time now. 50% hit rate with D6 S5 No-Save hits. Not too great for whittling down RnF, but wonderful for taking out many points from bricks of heavy cavalry.

Bolt Throwers, meanwhile, are just as accurate as a Dwarven BT (without an Engineer). You use them in numbers, either two teams of two or three teams of two, and remember that you'll only get 2-3 hits a turn statistically. You pick your targets carefully, so that those 2-3 hits a turn either tip the odds for one of your regiments or make up the BT's points more directly (ex: Plastering 3-4 Chaos Knights).