A few thoughts:

1. the nations you mentioned are some of the largest, most populous, and wealthiest nations in Europe (and North America). It's hard for a little country like Romania or Belgium to field the technologicaly advanced units of the Wehrmacht or UK forces--they have neither the manpower nor the material resources to maintain those forces. Nor would they have had the scientific/engineering infrastructure to rapidly develop new weapons to meet the needs of their armed forces.

2. More importantly, I'm not sure this question has an answer. The capabilities of each of the different armies changed dramatically as the war progressed. The Soviet army destroyed in 1942 was not the army that crushed Germany in 1945. Tactics, instructions, equipment had all changed in the 5 years of war. Likewise, the German army of March 1945--boys, old men, the sick--was a far cry from the crack force that invaded France in 1940. In 1940, the British bombers were suffering crippling losses on daylight raids without any result-by 1943 they were laying waste to whole cities on immense night-time operations. You get the idea.

With those caveats, I think there are two nations you aren't giving proper credit: France and Finland

France was defeated quickly, but you have to remember that in 1938 nobody thought that the French army would be out of the war in six weeks. Their army was large, they had extremely strong fortifications along the German border, they fielded a variety of armored fighting vehicles and tanks comparable to those in Germany at the time. They were considered one of the great powers of Europe before the war and the rapid fall of france was truly shocking--it's easy to forget that in hindsight.

And Finland, of course. During the Winter War the Finns were able to cut whole soviet divisions to shreds, and later helped the Germans seal off Lenningrad. I've not done enough reading on the subject to say whether that is a feat of Finnish strength or of Soviet weakness, so I'll leave that discussion for somebody with greater knowledge. T

Sources: IANAH, but this is pulled together from various books I've read on the subject over the years. Suggested further reading:

• Evans, Third Reich trilogy
• Atkinson, The Liberation Trilogy
• Clark, Blitzkrieg
• Todman, Britian's War 1937--1941 ( contains a really interesting look at technological developments on the british homefront in response to german threats.)
• Klein, A Call To Arms (covers US army planning and organization in the years leading up to WW2. Gives some idea of the scale of projects required to create a "quality" army in the 1930s.)
• Toll, Pacific Crucible (demonstrates how the US and Japanese armies/navies relative strengths changed as technological and doctrinal innovations are discovered on each side)

edit: Would non-native troops qualify in this regard? Everything I've ever read has suggested that exiles from Nazi-occupied nations who fought with the Allies comported themselves very very well. Free french, free polish, etc.