I'm pretty happy with nginx but I do mostly rails with that. For PHP, the general consensus seems to be fastcgi + lighttpd for high performance environments, but if you're just serving the same page, why not use apache + mod_cache. What improvements over mod_cache would wp-supercache get, you're just serving static files, so why not just add varnish or mod_cache for apache.
That said, serving small files from lighttpd is extremely fast. I set up one high-performance server for small files that served from ramdisk on a linux machine. At 15000 r/s with ab or siege I didn't max out the small instance on amazon. Sceptical about correctly measuring performance at that level though.
Lighttpd had a persistent bug where overloading FastCGI instance of PHP would eventually cause it to get "stuck" in a 500 error state, even after PHP had returned to idle.
That said, serving small files from lighttpd is extremely fast. I set up one high-performance server for small files that served from ramdisk on a linux machine. At 15000 r/s with ab or siege I didn't max out the small instance on amazon. Sceptical about correctly measuring performance at that level though.