I normally use EditPlus/Textpad (very light weight) if you check Zend.com, they have a studio (IDE) that you can use. I am not sure if its free
Download "wamp" and install it to use Mysql database and manage php files on your pc and dreamweaver to create php files.
I use Komodo IDE for most of my PHP editing now. Dreamweaver for CSS and Javascript though because the "intellisense" like function in Dreamweaver is much faster than it is in Komodo. I dont know what Adobe calls their "intellisense" but it all works the same. I used to use Zend Development Environment but Java is a nasty piece of malware I dont like to run too often, haha.
I use Programmer's Notepad. Its a nice little program that supports and actually helps debug a whole range of programming languages.
I wouldn't say "best" because it's based on a personal opinion, not fact I've used multiple programs and each have something I do and dont like about them. For the longest time I liked ZDE because of it's ability to increase development time of OOP applications. However, being that it runs on a java platform kinda killed it for me. I still use it every once in a while though.
I use linux and usually use Screem editor, but lately, with Ubuntu 7.10, it randomly closes (without saving). I since found Kate editor, which works pretty well. It has alot of nice shortcuts for browsing your files (no tabs, but it has a side menu which can expand with a list of open files). When I had Windows, I used php Designer 2008 and Context editor, both of which were pretty cool.
I'm using UltraEdit-32.. Perfect program... Actually an advanced notepad with syntax highlight, ftp, validation, tabs (multiple files open in one window) and much more..