Firefox. No other browser really compares to it in my opinion. If you have GDI++ on Windows, it makes the fonts look extremely smooth - almost like in Safari. I'm the first person to admit Firefox is incredibly bloated since it moved up to 3.0 and beyond, but with 4.0 we could see this being rectified. And even if it is bloated as well, it'll likely be compatible with nearly everything anyway, meaning that it's of no concern to me. I like the addons available for Firefox, and different skins are downright awesome. Personas are pretty sweet too, but I'm not really a fan of extremely bright colors in my Firefox styles, so I go for darker Personas. And yet, there are still plenty of them to choose from.
Chrome is a decent browser as well, but I find it really messes up font smoothing and makes most sites look unbelievably ugly. Try opening Safari on a Windows machine and put it beside Chrome on the same machine, at the same site. You'll see what I mean by font rendering. Chrome used to be my main browser, but once I found Firefox to be as good as it is, I can't use Chrome anymore. IMO, Chrome was at its finest when it was in the 1.0 phase. Once it moved up to 2.0, I wasn't as fond of it. I'll give it one thing though: it's incredibly fast, and if I need speed, I'll use it over anything else.
As for Opera, it feels completely unprepared for modern internet in some cases. I use a site in particular a lot, and it's also about videogames. Opera is not compatible with it, and that's downright worthless for me. I like the fact Opera is basically prepared for everything with all the addons, and it's still lighter than Firefox. What I don't like though is the fact that it feels slower due to the fact it alerts you to the fact it is downloading elements. It might not be slower, but it sure feels it when it tells you it's downloading the information. Though it renders my beloved fonts properly, so it's fine that way.
As for Safari, I have two views. I liked the older version better because it looked more like an Apple product. Plus, the buttons were like those from an OS X machine, so it felt like a Mac / Windows hybrid. With Safari 4 they dropped the Mac styling on the Windows version, and also the buttons. And with 3, I was prepared to sacrifice speed in some situations in order to get some great looks. With 4 I don't get great looks and speed is still lacking.
Internet Explorer is a browser I won't even mention. It does the job - getting to Mozilla's page and allowing me to download Firefox. Otherwise I don't bother. So, here are my rankings for them:
1. Firefox
2. Safari
3. Chrome
4. Opera
5. Internet Explorer