Najzaostalejšie regióny. Whatever this means in EU terms, it doesn't sound very positive or complimentary. It is typical of the Slovak language to be direct, not to pull its punches, and "zaostalý" suggests something that has "remained behind" in the forward movement which is progress or competition.

There is an English word "backward" which is even stronger, because it suggests movement in the opposite direction. Applying the principle that people get "labeled" (the Slovak idea of "nálepka" is appropriate, expressing the idea of a reputation which "sticks" onto people like glue), the people living in a region that is called "backward" could lose hope, become apathetic and fatalistic, so it might be better to use a different word which suggests the possibility of change, potential for improvement, and this is the idea behind the term "underdeveloped regions".