"What's the best language" is a sure fire way to not get any useful answers(because you'll get all possible answers) from a crowd of programmers, IMO.

What's "best" for what? Does the safety critical system need to be fast and respond to real time events in real time? Or is it safety critical but is allowed to be slow? What kind of hardware is it running on?

Depending on your hardware, the only safety critical language may be assembler, so you can check every single instruction and make sure it's precise.

Having said all that, Apollo 11's code was written in a meta-language, then compiled to assembly instructions, then hand-compiled into computer code. You don't get any more safety critical than taking the first men to the moon.

http://googlecode.blogspot.com.br/2009/07/apollo-11-missions...