I had created a post here regarding a Varnish plugin we wrote for cPanel over a weekend codeathon (http://www.unixy.net/varnish). Based on benchmarks, Varnish+Apache came out ahead of Litespeed. But benchmarks are not always conclusive since user traffic patterns are difficult to reproduce in a benchmark environments.

The feedback we've gotten so far on the plugin is very positive (google if interested). Nginx can do caching but not with the same flexibility as Varnish.

Regards