The most recently selected stamp does not become the default selection the next time. However you can select a stamp (by clicking on it) and then click on the stamp annotation dropdown and click on the link "Add current stamp to favourites." and then it will be displayed in the list of stamps that can be seen on the top.
Hope that helps.
So, what your saying is that this function, the ability to select the most recently used stamp by clicking the stamp tool, has been removed, in spite of the fact that Acrobat's Help pages says it can be done (Using Acrobat X Standard > Collaboration > Commenting > Apply a Stamp > Step 1 > Second bullet). I wish they would put it back. It was quite handy.
Perhaps I should adjust my request. Do you (or anyone else) know of a way to use the same stamp several times in a row without having to choose the stamp from the stamps list? For example, if you needed to put a check-mark stamp in several places in a document, it would be a pain to have to go back to the list each time. Is there perhaps an undocumented feature (maybe holding down a particular key while using the stamp) that would allow for multiple stamping?
You do have the option to keep the stamp selected. To do so, first display the Properties toolbar (Ctrl+E), then select a stamp, and you should see the "Keep tool selected" check box in what is now the "Stamp Tool Properties" toolbar. Select it and you can place the stamp multiple times without having to reselect it.