Apache buys you two things: rock-solid, time-tested stability, and millions of words of writing both online and in print about administration and tuning.

Lighty and Nginx are both (like thttpd) leaner, potentially faster, and "sexier." Neither has the proven stability or community that Apache does, and neither is going to help much if your bottlenecks are at the application layer, rather than just shooting raw bits over a network cable.