Favorite Plant Monster? Why, its the Shambling Mound!
According to wikipedia, the beloved Shambling Mound (which obviously deserves its own entry):
Shambling mounds are usually about six feet tall, with an eight foot girth, when they stand erect. Most weigh around 3,800 pounds. Their brains and sensory organs are located in their upper regions. In their natural environment, shambling mounds are almost invisible. They can move through water easily, and have been known to creep into camps in their range at night to attack. Stories tell of shambling mounds moving about in electrical storms without flinching from the frequent lightning strikes, even direct strikes."
So the standard Shambling Mount is six feet tall, but with an eight foot girth. That's a wide load right there. I also wouldn't have guessed their sensory organs are in their upper regions, you know, where their head is. I don't know why they are so immune to (and in some versions healed by) lightning, must be some reference from some old story I'm missing, but whatever. I picture shambling mounds to be more like a cross between an Ent and a giant Swamp Thing, but way dumber. They basically are gigantic heaps of swampy vegetation that walk slowly around forested areas and beat the hell out of anything that's not a druid. When you play a druid that moment when you can finally summon a shambling mound is a special time, because you know you can now fully commune with all forms of nature, even the sentient plants... and command them to kill dudes.