What is the best pesticide to use on tomato plants?

I'm growing two tomato plants in seperate containers. On one of my plants some type of bug is eating the foliage at the bottom of the plant. They seem to be leaving the actual fruit alone, but it's making the plant look awful. I found a black looking caterpillar on the plant that I knocked off and killed. I'm thinking that might be the problem bug. Also, I've tried insecticidal soap but it doesn't seem to be helping.