Generally, I've got a strong sentiment towards Austrian WWI uniform. I like 1912 Swedish and 1930s Czechoslovak uniforms, too - probably they are not extremely spectacular, but highly practical, every detail well thought of, and in my eyes they look smart. Speaking of WWII, here are my personal favorites:

Captain, Lancashire Fusiliers

RAAF Flight Lieutenant in BDU jacket and visor cap

Captain, United States Marine Corps "Dress Blues" uniform

Captain, Red Army Air Force (1940)

Centurione, Italian MVSN/CCNN "M" Battalion
(All pictures from the Captains' pages of Diggerhistory, of course)