Hi Jaap,
Personally I think the last one is the best, but I would be tempted to move the back left one in a little.
Before that however, I think you need to sort the substrate out as the lack of definition in it is detracting from the definition you're trying to achieve with the rocks. Have a look at some other iwagumi scapes for inspiration, but most would argue that it looks best shallower at the front and sloped deeper towards the back.
Hope this helps for starters.
Ben