Dior show by DIOR, is my fave for thickness and length. At the moment i am using a Clinique mascara and i love that too. I have tried maybelline and rimmel, they are good, but i definitely think that you get what you pay for with the more expensive brands like Dior and clinique. They are worth every cent.
If i am interested in a new mascarra, i usually look on ebay. People sell the trail sized mascarras (brand new & unused) they are cheap, so its not a waste if you dont end up liking that mascarra.