It depends on what you intend to be your "next step".
For landscapes,my favourite is my Sigma 17-35mm(expensive,but worth it).
For wildlife,it's my Sigma 175-500mm(even more expensive,but nice to use).
For closeup/macro photos it's the Cosina 100mm macro(cheap,but I like it)
But the lens I use most often is my 70-300mm.
Canon do an Image Stabilised version(which cut the amount of film I waste because of camera shake by over 90%!),but even the "conventional" version is useful-especially with a 2x convertor.
Also,a good tripod is ALWAYS worth having!