I think they are all in the same league. I own a full bottle of l'Eau de l'Eau and one sample of L'Eau des Hespérides. They are both marketed as "colognes" (and come in different bottles from the classic rectangular Diptyque bottles) but their concentration (80° vol.) is the same as L'Eau, L'Ombre dans L'Eau or l'Eau Lente.
I get more than 7 to 8 hours longevity out of l'Eau de L'Eau, which is extremely decent for something marketed as a cologne. I am not so much enthusiastic about L'Eau des Hespérides. There is a strong, fruity note that I also detect in L'Ombre dans l'Eau (blackcurrant bud) that I find too sweet for a cologne. I like my citrus scents to be dry and tangy.
I also own the hard-to-find Eau d'Elide but it hasn't really grown on me (orange and lavender).
None of the above is a safe blind buy and they all require to be tested on the skin. If I had to establish a Diptyque shortlist today it would be quite different from what I own:
I would buy L'Eau Lente
I would replace L'Eau de l'Eau by L'Eau (the first Diptyque fragrance)
I would keep L'Autre, there is nothing like it (lime and cumin)
I would get rid of L'Eau d'Elide
What I have learned with Diptyque is that I love lemon, lime and grapefruit notes but for some reason I dislike orange and overly sweet fruit notes.
Diptyque makes excellent spicy fragrances in a sort of pot pourri style which is very wearable and definitely not a home-fragrance style. Their citrus and colognes are original and differ from the more traditional takes of Guerlain and Nicolaï on the genre.