This depends on your application.
If you are serving static content, the most benchmarks show systems like nginx and lighttpd to be faster, but if you are serving complex applications, apache or litespeed may be required.
Too often, I see people look for the "fastest" server to power there web site. Somethings that I find people often overlook:
-At your traffic levels, the server choice is largely irrelevant.
-The difference in speeds between servers may be trivial compared to the gains achieved with better code.
-If you have the wrong hosting infrastructure, changing HTTP servers is merely a patch for bad design.
Finally,
Test, test test. The results may surprise you.
I had a client that insisted we install something like tux, lighttpd, or nginx for his site to "speed it up." I explained at length that the HTTP server was not the problem but that the disk subsystem was the issue. After much debate, I relented. Installed 3 different HTTP servers with no improvement.
I asked for a 15K SCSI disk to be added to the system - problem fixed.
So unless you know your HTTP server choice is an issue, there are probably more important reasons than speed to guide your server selection.