Panerai - I've got two and think a collection could always do with one. Bit also depends if you've had any in the past?
Monaco - not a fan
Milgauss - one of the current fav Rolex. Especially the black GV model. Tried many times but not owned one yet, timing never right. But one I really do want to try.
For me personally, it would be the Milgauss, but only because I have Panerai covered.