"Things" is pretty broad.
Thinking of software, probably early "beta" Gmail. There was so much about the email experience that was improved by a stable web client, with what felt at the time like unlimited storage.
Thinking of hardware, I've never been much for Apple products, but the iPhone (particularly early models) is undoubtedly a design marvel.
Things has a really good open-source clone called Planner[0] that I quite like. Now, if only they'd package it some way other than flatpak...