Is the polonaise Nr1 Opus 40 the easiest piece of music of the all Chopin's Polonaises?

I would say that it is probably the easiest of the polonaises and its repetition makes it easier to learn. It is the one I learned first but it is by no means easy to play well. The first four polonaises, being opus 26 no. 1 and 2 and opus 40 no. 1 and 2, do not contain any severe technical hurdles and should be manageable by a post grade 8 pianist. All the others are impossible to anyone but the most talented of piano players.