Actually, not much of a difference. The first sentence is closest to the meaning: Heilong Jiang is one the northernmost provinces of China. While the second one just indicates that Heilong Jiang is the northernmost province of China.

This is apple. This is an apple. 对吗？

两个表达的意思类似，有“一个”，表示语气较弱，强调多个省中的某个特殊的省，第二个句子更强调“最北”这个词。

The difference being: Heilongjiang is a province that is northernmost of China. Heilongjiang is the northernmost province of China. Almost identical meanings, but differ in specificity.