What is the Best Camera Lens?

I am an amateur wedding photographer on the side. I am searching for the best lens that I can use for a variety of environments. I am photographing a friend's wedding in April on the beach, and shortly thereafter a late night wedding in a banquet hall setting. I need a lens that can pull duty at both. Being a beginning I need gigs to buy more equipment. I am currently shooting with a Canon Rebel XTI with a 18-55mm lens that came with the camera body. I have a Tamaron 70-300mm lens that i think is only ok. Please help.