Started out with Ruby on Rails, changed to Python (pylons) + mako + toscawidgets + postgres for more flexibility. Considering giving Common Lisp (Uncommon Web) a try, but the documentation scared the heck out of me.
Rails, as stated by its creators, is opinionated software, so you're stuck with their preferred way of doing things, which fortunately is not too bad as of now. So if you're a control-freak like me who also wants to squeeze more juice out of your CPU cycles, then go for python. If you want to put your trust in a well-integrated framework of components cherry-picked by smart folks, then go for rails or django (also python, but a little less flexible).
Here's a fairly up-to-date overview of python web dev options: http://jesusphreak.infogami.com/blog/vrp1