The best solution for WordPress optimization we have discovered is W3 Total Cache plugin. It can be found at http://WordPress.org/extend/plugins/w3-total-cache/ , however for best results it should be configured properly.
Here are settings that achieved best results during our tests:
1. Page Cache = Disk (Enhanced)
2. Minify Cache = Opcode
3. Database Cache = Opcode
4. Object Cache = Opcode
For Opcode you can install any of PHP caching engines like APC, XCache or eAccelerator .
However, you should understand that website speed depends on many other things, like mysql database speed, number of static files per page, different plugins, etc.