A few of these are good suggestions but I can't live without C-v, whether I am already in visual mode or not. Block selection may not be used that frequently but when it is, it's a lifesaver.
I use block selection so often that I've switched the mappings in my vimrc. I have v mapped to visual-block selection and ctrl-v mapped to visual selection. It's very rare that I miss normal visual mode as I use block and line selection more than anything.
