Django and Python. Because they're just that good.
But, really, use what you're good at. If you're working on a startup, do you really want to spend valuable time getting up to speed with a programming language that you don't know and are only learning because random people on the Internet told you they were using it?
"If you're working on a startup, do you really want to spend valuable time getting up to speed with a programming language that you don't know"
I did that.
It probably hurt the startup a bit - we'd be moving much faster if I didn't have to learn the technologies as I went along. Thing is - the web technologies I really know well consist of PHP and JSF, and I know them well enough to know that I do not want to be programming in them if I can help it.
I figure that any startup will be taking up a good chunk of your time, and you might as well make it at least semi-enjoyable. I'd much rather put in the up-front effort to learn Python and have a tool that can grow with me than curse PHP's naming conventions or lack thereof for the umpteenth time.
But, really, use what you're good at. If you're working on a startup, do you really want to spend valuable time getting up to speed with a programming language that you don't know and are only learning because random people on the Internet told you they were using it?