Well, I generally make pages that work in Firefox before any other browser. Apart from Opera, Firefox is most likely the most standards compliant browser out there. So if I make page that work perfect in Firefox, all other browsers on the Gecko engine (such as Netscape Navigator) will handle the page round about the same as Firefox. Although, I then test the page in Opera, which will usually display it nearly the same as Firefox, except for some spacing issues between elements, fix all of the spacing errors up, then go to Internet Explorer. Internet Explorer will then bring up a whole range of problems depending how the page was setup. For instance I generally have more problems with CSS block-level layouts in IE than I do with tables, becuase not much can go wrong with tables. IE doesn't support quite a lot of CSS 2.1 at the moment so it's mainly with positioning and pseudo classes that the problems arise.
Overall Internet Explorer is the worst for me, although IE7 looks quite promising in the way of W3C compliance, but I still don't think they will get it perfect. What I'm worried about with IE7 is its compatibility with older OSs. If Windows 95 users can't download IE7 and get it running comfortably on their system, then I will have to continue making pages working in IE6 to satisfy the 0.01% of the population on Windows 95; and the rest of the people who don't want to upgrade to IE7 (who I will most likely class as weird or stupid).
See how compliant each browser is:
Acid2 Browser Test.