Sometimes going in the opposite direction works when picking ingredients. Since you already have an overwhelming amount of cucumber flavor (or aroma) from the cucumber itself, picking a gin with complementary contrasting botanicals might work -- maybe a classic juniper-forward London dry?
I used Hendrick's gin for its cucumber notes.
Until the price went from "too high" to "insulting", Plymouth was my choice for pairing against cucumber.
It's funny that you both reacted to my gin choice. I can't say that I disagree with you. I had a little debate with my husband last night on that very subject but I decided to let him have his way...
Here is the whole story. I made quite a bit of this cucumber lemonade last Saturday so we had a chance to try it with various gins over the weekend. Initially, I very foolishly reached for a bottle of Junipero, my motivation being that there were only a few ounces left and that I wanted to kill that bottle. Talk about contrast! I love Junipero in some other applications (Negroni, Cin Cyn, etc) but it was absolutely hideous in that drink (definitely not "complementary"!).
Then we tried the cucumber lemonade with Tanqueray at our friends' - a great match, perfect balance, really great.
Last night I discussed our options with my husband since we did not have Tanqueray and Junipero was out of the question. I was considering Plymouth or Beefeater. He wanted to try it Hendrick's, which got lost a little in the drink due to lack of contrast. It was, of course, much better than the Junipero, but I agree with both of you, not the best match.
Next time I would use Plymouth or Beefeater. I was thinking that Bols genever could be also an interesting option with smoky flavors that could add another dimension to the drink, similar to a John Collins.