This one's a bit unfair, yeah? I get the natural theme of choosing amongst elementals and plants, but the two things offer quite a range, and it becomes hard to choose. And so, I'm going to cheat on this point and choose one from each category!
Representing the elementals, I think I have to go with the classic earth elemental. This guy combines great movement ability with staying power in the form of high hit points, as well as the physical might to stand toe to toe with some fighter types when damage is concerned.
A summoned earth elemental is a great boon in holding passages or harrying spellcasters that cannot fly, or making them grapple said spellcasters to prevent them from flying. Earth elementals, however, are just as fun and useful to utilize for the GM as they are the players, especially if their earth glide special ability comes into play.
In the plant's corner, we have the assassin vine. I admittedly do not use many plants (though I tend to roll for shambling mounds often enough in my Wilderlands campaign), but one that I have used over the years to measured success is the assassin vine.
This is a nice "trap beast" to use for parties who like to retreat from the dungeon every 15 minutes, camping in the woods between forays into the deeps. If only they had been more attentive, they would have noticed the creepers trailing up the trees nearby were deadly flora.
The assassin vine is also easily scalable, meaning it can be a threat even when the PCs start ranging into mid-levels.
As a final note, there aren't really any super neat images of either of these things on the net, but there was an earth elemental on some person's blog that reminded me of an old Rock Lords Go-Bots.