This is a pretty simple answer for me: wilderness.
When it comes to interesting encounters, nothing beats nature. The wide variety of ecosystems alone gives the DM a wealth of possibilities, and because the PCs are out in the open, they have a greater sense of freedom. Harsh climates provide their own challenges that can overcome a low-level or ill-prepared party. Storms and weather conditions can give a wilderness encounter extra flair, or can just serve as a nice excuse for the characters to have some down time.
Deserts are a particular favorite of mine. I’m not sure if it is just that everything in a desert wants to kill the PCs or what, but the inhospitable environment of a desert makes for some great adventures.