Honestly, everything that I don’t have from the Halo and any terrain pieces in general. Also just about all of the CoD heroes, just for their gunbelts and other accessories. I’d love to just go ballistic with accessories for Winters and the gang, but Halo sets don’t come with accessories like that (which makes sense, the characters in-game don’t have them, so...) But if I had to choose a few, it would be these:
Original Pelican
Phantom
Aerial Ambush (2019)
Flood Siege
Countdown
Floodgate/Flood Invasion
Scarab
Spirit Dropship
Vulture
and most certainly the unreleased Firebase.
All but the Firebase were just too big and expensive.