Oh, we all have are our favourites. So one person's "Must-have" will be someone else's "Don't want".
But, for me, it is pretty much everything by Aris, Tanks a lot's buildings, Juju's UI and Mord's Unit portraits. I also have a wealth of uniform mods but, some of them never made it from the original Repository over to CMMODSIII
As for music mods I make my own.