It is tough to narrow it down;
Some honorable mentions:
ZT0600 - wonderfully large, beautifully crafted, RJ Martin design.
ZT0888MAX - In house design, technically innovative, world class machining, and Maximet.
ZT0560CBCF - A classic in the standard design, with CF and a S110V composite blade...all the good stuff.
ZT0562CF - An evolution of the 560, a little smaller and more refined. One I carry often.
ZT0392 - Top of the Hinderer Collaboration Heap in a LE. What's not to love.
My choice for number one if I had to choose today (might be different tomorrow...)
ZT0454 - the original. Stunning Sinkevich design, innovative sub-frame lock design, with 3D machined CF, 3D machined TI, one of the best integrated 3d Ti clips ever made, and finally that spectacular three-piece composite blade with D2 steel for the edge and spine. ...the top of a very elite heap.
best
mqqn