I'm not a 100% sure if I should be posting this here but where else can I post it (definitely not Server Fault or Super User) so hopefully it's not too inappropriate.

I am currently developing a script that I hope to release as a plugin for wordpress and other open source content management systems. The script's purpose is to allow web designers to attach stylesheets and javascripts dynamically according to which browser the visitor is using. So if their site looks a little odd in Chrome for example, then they can attach a css or javascript hack just for that one browser. There will be some more features to the script as well but that is the main purpose.

Anyway, I would like to know which browsers to include support for, for that I need to know which one's gives designers/developers the most trouble. Not just the name but also the version (i.e. Internet Explorer 4). Many thanks in advance!

IE6 is by far the most problematic of all browsers. Though it's use is declining month by month, it's still widely used. All IE browsers always suffered from display issues. Sometimes margins are not what they were set to, because you need to go down the css hierarchy to set all margins for it to understand what's going on. You shouldn't need to worry about anything below IE6.