I've been using the twenty twelve theme for a while, but I think I actually prefer the "sandbox" theme by plaintext.org that I used on my wordpress.com blog. It's quite a minimalist theme, and twenty twelve just seems a bit "generic" in appearance. Im not sure why the wordpress.org version of sandbox is one version "older" than the wordpress.com version. There are warbings that Sandbox has not been updated for a long time, but it still seems like a good theme.