@Seakad
As far as I know, there is no way for Max to automatically select one of the choices of bitmaps in the dropdown list as the default when you open the Unwrap UVW Editor. This has to be user guided.
In theory, with a few lines of maxscript you can select one of the maps by order of appearance in the list, but you have to know the number of the map in the dropdown list. And by the time you have opened up the list to check this, it's just one more click to select it, so that doesn't seem useful to me. However, typically the diffuse or base_color map is the 3rd map after the 2 checker options on my installations of Max, so if you want the 3rd map to be selected you could run the following lines of maxscript (from the Maxscript Editor NOT the Maxscript listener):
uvwMod = $.modifiers[#Unwrap_UVW]
uvwMod.unwrap.SetCurrentMap 3
Copy and paste the above lines into a new Maxscript Editor Window and select Tools>Evaluate All or highlight/select the lines and select Tools>Evalaute Line Selection. Like I said, I don't see how this is particularly useful but possibly I am not fully understanding your issue and if so please clarify. Maybe a screenshot of your dropdown list would be helpful.