Difficult choice. I'm definitely a CTer, but in terms of which book I have found the most useful?
As a confirmed gearhead, I'd have to say perhaps LBB3 (since it defined the entire universe in 44 A5 pages), LBB5 (I've designed a lot of ships with that book and IMTU it made LBB2 pretty much obsolete), or Book 3 (Equipment) of Striker, which created every other vehicle and weapon system in the universe, effectively replacing LBB1.
I'd say 90% of MTU is designed from these three books.
Following close behind is the Character generation system from MgT, particularly the brilliant point-buy system on p40 - a couple of paragraphs that punch well above their weight for a GM trying to create a balanced game.