Having worked in PHP, Java, Python, C, and to a much lesser extent Assembly, I would have to say that while Assembly is very powerful, I find it too hard to use for it to be viable in a production environment. C is next on power, since "everything" has C at it's core. Python is almost a good as C, since it is built off C, it can theoretically do anything C can, just slower. PHP and Java are in a virtual tie, at least to the extent I have used them. Power isn't everything though, as it takes a good balance of usability and performance to make a good language. To that end, many would consider Java to rule supreme, but personally I feel C does a much better job. I will admit that part of that comes from the pain I went through managing "strings" in C when I took my class on it earlier this school year. I learned it through much hard work, and I don't want to just leave it behind.
All in all, for general purpose programming, Java and C are essentially tied.