I am such a generalist that it's hard for me to say any one type of terrain or vegetation, though if I had to choose only one, it would have to be the high mountains above treeline, likely the Sierra. But I really am deeply in love with nature in all its forms, with biology and geology alike. A friend of mine kids me that every trip I ever come back from, no matter where, I always rave about how it is one of the best trips I've ever done Another buddy of mine, who I've done quite a few high alpine trips with, just never understands why I also want to do trips in Big Sur, or at the bottoms of canyons, or in deserts, or why I have to stop and look at bugs and flowers all the time. I suppose I am most dubious of flat places...but I loved backpacking through dense tropical rainforest in Costa Rica: no far-reaching views and it was pretty flat, but that was one of the best trips I've ever done
Speaking of having a love that comes from where you first started backpacking, I grew up in the Missouri Ozarks and from those oak forests, I have always carried a certain affinity for the oak forests of California, although my preference in that regard is more a mixed oak and conifer forest. Something about Big Sur stirs up some feelings of warm familiarity in that regard (not that Missouri is much like Big Sur:)