The best language for a startup is one the founders know the best. Every other discussion will at best make a 1% difference.
Chewing through TBs of data every hour? Probably not a good idea to use Ruby.
Doing a bunch of really heavy math? Probably not a good idea to choose PHP.
Doing a SaaS service? C++ probably isn't a good choice.
There are some really good general purpose languages that can fit most circumstances (Java, .Net languages, Go). However, I'd be careful to say that you could apply them everywhere.