I personally think every possible subject to photograph has been beaten to death a million times, revived, and then beaten again...I very rarely fave Photographs, but they are often nature ones when I do...Kinda sucks since so many of the fantastic digital artists I watch are "converting" to photography, which lets face it - is easier than making their brilliant works from scratch
I love nature because it's so amazing and varied in its forms and colors (especially its colors!), but even more, I love a mix of street shots and architecture. I love seeing derelict buildings, or those under construction, or just strange/average buildings with an intriguing emphasis (again, perhaps with color).
It's even more impressive if people can be involved. Maybe it's just my city-girl-ness, but I like watching people on the street - how they walk, how they talk to one another, the things they carry, the rush to the subway, frustration at the lack of cabs or buses, children especially. If an image can capture these movements and feelings in one, with people and their surroundings, I can't help but love it.
Nullermanden does great people-in-cities shots (look him up!), but by far my favorite photo is [link] . This captures architecture, people, streets, and even some nature and lots of color.