This is a tough one. There are so many great looking uniforms throughout history that to try and pick one or two is difficult. If I had to choose, I particularly like the militaria of the Roman republic around the time of Julius Caesar. When you had a high ranking soldier with all that regalia and colo and the red crested helmets, I think that is the best. Then again, I did study Latin in high school for 3 years.
If I had to make a number 2 choice, I would say the English uniforms of the medieval period with all the colors and coats of arms.