Bring only one app to front after "show desktop" applied
I use this script set up as a folder action. Basically, whenever I make a screenshot, this script activates, opening the folder I use for screenshots in the Finder, and opening the screenshot in Preview for editing. Copy it into Script Editor, save it as a script to ~/Library/Scripts/Folder Action Scripts/some_name.scpt and then use the Folder Actions Dispatcher app (available through spotlight, from down in the /System/Library/CoreServices folder) to attach the script to the desktop folder or whatever folder you prefer.
on adding folder items to this_folder after receiving these_items
    repeat with this_item in these_items
        set item_path to POSIX path of this_item
        tell application &quot;System Events&quot;
            set this_disk_item to disk item item_path
            tell this_disk_item
                if its name begins with &quot;Screen Shot&quot; and kind is &quot;PNG Image&quot; then
                    -- this opens the containing folder in the Finder
                    open its container
                    -- this opens the screen shot in Preview for editing
                    open it
                end if
            end tell
        end tell
    end repeat
end adding folder items to