I've got:
Emperor's Arsenal, and if you want to look at the same weapon 5 times at 5 different tech levels, it's ok, I think it could have gotten consolidated, tho. Lot's of Weapons, and a nice dissertation about Nukes in Traveller.
Emperor's Vehicles, it's got one example of just about every TYPE of vehicle that isn't a Starship or Small Craft (even has Fighters and Bombers), including (my favorite) a "Personal, Grav" vehicle which is the Traveller version of a Star Wars speeder bike.
Central Supply Catalog has lots and lots of Milieu 0 specific equipment (focusing on Fusion+ a lot).
Aliens Archive has maybe 3 aliens I've found useful - kinda disappointed in that one.
Psionics Institute has just about everything you've ever wanted to know about Psionics and where to learn them at (except for the "new" psionics that were in The New Era).
Naval Architech's Manual has a ton of room-specific deck plans (i.e. there's a deck plan for a small, medium, and large bridge, a small, medium, and large engine room, etc...). I just liked looking through this one.
And some day, I'm going to find me a copy of Starships.
Hope that helped,
Scout