My personal most-feared are:
1) Gunnera. This stuff is like really really fucking giant Rhubarb but with big spikes on the underneath:
How fucking big?: http://www.worldplants.com/gunnera.htm
2) Sunflowers. I just hate how they actually look like a really big scary fucking eye. And it's looking RIGHT AT YOU. AAAAAAAAAAHHHHHHHHHH! GET OUT OF MY HEAD! GET OUT OF MY HEAD! They turn to face the sun. THEY MOVE! THEY MOOOOVE! And they're plants. AIIIIEEE!
Trees are okay 'cos they're not oversized and are meant to be that size, see?