Here is the updated CSS, which should work in most of the cases:
z-index:1; /*This should be larger than rdContent*/
I should mention that there is no universal method for making a RadMenu which is inside a RadDock control to appear on top of all other RadDock controls. This is because the browser calculates the Z-Index relatively to its DOM tree, e.g. RadMenu's Z-Index is relative to its parent, e.g. RadMenu could be on top of all children of its parent RadDock, but cannot be on top of the children of another RadDock, which has higher Z-Index of RadMenu's parent. If we make all RadDock controls to have equal Z-Index, the browser stacks the html elements by their order in the DOM tree. All this means that the RadMenu, which inside the topmost RadDock control will be displayed over the other RadDock controls, but their menus will appear below the RadDock controls which are with higher Z-Index.
All the best,
Valeri Hristov (Senior Developer, MCSD)
the Telerik team