Currently I am using Apache, I've been using OLS2 for a little while but I saw that (in my eyes) the memory footprint of OLS for PHP processing is quite high.
Since there isn't good service side caching available, you will be forced to use caching on the application side.
Wordpress does provide the litespeed caching technology, although it works decent. I prefer server side caching a lot more.
So I've decided to switch back to apache combined with php-fpm. Tuning the php-fpm a little bit, combined with application caching it gave me a better memory footprint for php processing 🙂
When nginx is out, I will definitely switch to nginx and hopefully they will introduce fastcgi caching too!