I EDC'd my ZT 550 a long time, and it kicked everything else out of my pocket until I just stopped loving it out of the blue. I was initially concerned with the bulkiness, weight, and overall size, but it carries amazingly. It feels like a much smaller knife than it is. If it fits into your pockets and you like a sturdy knife, the 550 is a superb choice.
If you want something that carries better, the 801 is awesome. It is a little less bulky in pocket, and has the sweet KVT bearing system. I had to dial in the lock bar tension for the detent, but now that it is perfect I EDC this knife pretty frequently. It flips amazingly, and the blade profile is a superb EDC blade. It would be pretty hard for me to be happier with the blade unless I found money wedged into it.
ZT knives are excellent. Their CS takes care of every problem, and the knives are pretty low maintenance by design. I really love ZT.