Depends on what you need. If you use PHP you can get the most support on this forum so if you are new then go with PHP.
I would say in this order PHP ASP Cold Fusion others. PHP is the language of the people, and is the best )IMO) for a number of reasons, namely availability of cheap hosting, widespread support forums etc. ASP is however the taught language of acedemia and as such is catching up quickly, it is also seen as a microsoft windows language with all the MS foibles. Cold fusion is a bag of bones unless you are a true pro and can write beautiful code, when it is great. Cold fusion hosting is expensive though because everything has to be paid for, plus there are also compatability problems with different modules installed on servers, making it difficult to move hosts. CF iis also resource hungry, and because of the expense of the server software, often times hosting companies will place too many sites on a server which slows down response times. just my $0.02
My favorite Server Side Language is Java. I use Perl for most of my personal sites. I think Ruby is really awesome and is coming on strong.