Why not begin with 25/1? It really frees up the arms, the body, and initiates one into the physiological vocabulary of the composer. I think it is the prerequisite to the other études and ought to be learnt first. It is not a question of the easiest étude, but rather of the best one to learn first. On the other hand, you might examine your repertoire and see which étude most benefits the pieces you are presently studying. On the third hand

, you might just see which interest you most, as you will eventually learn all of them anyway.
A side note, to the masses: I read today that Neuhaus advocated learning 10/2 and 25/6 simultaneously. Interesting, isn't it?