Frankenstein, or in D&D speak, a flesh golem. This is my answer for what is my favorite elemental/plant/construct. I like the idea of including classic concepts into my games, and a flesh golem is just that.
I’m not really being fair to the other choices for this category, but I also don’t have the same love for elementals or plants. That’s not true. Elementals are very cool, and recently I’ve been considering narrative ideas to bring them into my own games.
The reason I like flesh golems though is that I find it just allows me to bring elements of fiction into my games that I love. Tortured characters, freakish monsters, and anything that contributes to the first two.
Also, I love dark fantasy and horror elements in fantasy stories. Infusing a fantasy adventure campaign with a horrifying creature like a flesh golem, and of course, its creator. Granted, this allows certain kinds of story lines, but you can’t really force the issue too much, you know? Regardless, this kind of monster is very cool.
With golems of any stripe, it is necessary to have more than just the golem. It adds a level of complexity to the story when you know you’ll probably be dealing with more than the initial threat. With plants and elementals, their reason for existing just isn’t the same.
Same thing with plants. In many cases, plants are basically just furniture. They exist in the space, but they don’t really require other people. They can, but not always. Often, plants are, at least for me, environmental hazards. So, long live flesh golems.