I like the 1st and 3rd one, the 3rd being my favorite as it is a custom piece. The 1st actually has a lot of work done to it when put side to side with a normal DP and you could even do more with the hands and arms, you can also give him a Tech Priest like backpack in addition to the jump pack he has now.
The 3rd one is a full custom bit build that looks pretty nice and not to hard to do. Random Obit, Tomb Spider, and Orc bitz pretty much.
The last one to me does not fit the theme of the Iron Warriors, kinda reminds me of a Balrog. The 2nd ones, TBH looks stupid, its like he has a Surefire flashlight strapped to his back.
You can also start with a dreadnaught based model and move from there as well.