We can't even conquer Afghanistan...we sure as shyt can't conquer Iran, China, or Russia.
As for a winning a war...who even knows what "winning" would entail?
I guess Iran would be "easiest" in a non-conquest-motivated war b/c our air force and navy dwarf theirs...Russia's and China's too, for that matter. Plus we have the advantage of having military installments all over the damn place.
If the goal were conquest...maybe Russia would be easiest? Least dense population, closest to America with regard to values, culture, and traditions, newest and least stable government institutions, etc... You could debate all those, of course.
Still would be a fukking nightmare that we would ultimately fail at.