No framework is best. One framework performs best on a certain situation. In general there is no best option. For example, Cake PHP is best for faster development. But takes time to optimize. CodeIgniter is best for its fast loading. ZendFramework is best for creating very large maintainable websites. You can not generally chose one and say its the best.
My vote for codeigniter too. No particular reason its the only framework I learned to work with recently. I'm planning to learn cakephp too.
Use CodeIgniter. It's extremely easy to learn and it works very well. Also, it's very extendable if you'd like to add some extra power to it.
I'm not sure how to tell you why Yii is the best but I enjoyed it. The simplicity on how to build an application is unreal and more simple than CakePHP (IMO) which I tried for a week or two but found it too complicated. I was a little like you all I used was PHP, I started with a fresh notepad++ file every time. Frameworks don't offer any new functionality it is designed to simplify things. So my advice is install some frameworks maybe 2-3 and have a go at each for 1 week or 2, reading their API and doing some tutorials. The one you find easiest to follow then that's the one you stick with. I tried CakePHP and didn't like it but there are thousands of others that do. I tried Yii right after and loved it while thousands of others don't. If you want my opinion give Yii a go and I don't think you'll regret it.