Also going by category:
Book:
High Guard definitely. This book had everything -- advanced char-gen like
Mercenary only better (gotta love Flight School!), incredibly versatile, crunchily detailed, and yet still simple enough to do without a spreadsheet ship design, and even a statistical fleet combat system to watch all those bad boys smash each other without having to devote an entire evening to it. Yeah the luster has dulled a bit since Chris Thrash convincingly argued that those big ships actually wrecked the OTU setting, but we didn't know that at the time, and, by golly, they're just so
kewl!
Supplement:
Traders and Gunboats. Love the deckplans (even if the Empress Marava is about twice the proper size
), really love all the chrome and color and detail (especially the air-filter on the Type S!), just plain love the whole thing.
Adventure:
Twilight's Peak. The Epic. Absolutely everything you could want in a Traveller adventure all in one little book. Yeah the Sky Raiders trilogy and
The Traveller Adventure have more chrome and detail, but they've also got way higher page-counts.
Double Adventure:
Chamax Plague /
Horde. The former is IMO the perfect "introductory" Traveller adventure. The latter is a 1950s Monster B-Movie come gloriously to life. Gotta love the Keith brothers who, more than anyone else, understood that adventures should be "Adventures!"
Game:
Azhanti High Lightning rulebook. Fast, simple, and deadly. My all time favorite rpg gun-combat system.
Special Supplement: Should this really be a category? Would anyone say anything but SS2?