Firefox!
I switched to using Chromium (the open source version of Chrome) for a few months, but recently switched back. I do love Firefox’s extensions, but to be fair, Chrome/ium does have nearly as many good plugins/extensions too, including AdBlock. (Though I noticed that Google’s own ads somehow sneak through on Chromium.
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Sure, Chrome starts up faster, but once you’ve got the thing running, I don’t really notice a difference.
There are some really geeky things I like better about Firefox: opentype font automatic ligature substitution, SVG images with transparent backgrounds, mozilla-specific CSS, and some other minor things. I find that Java works better, although slower, than with Chrom(e)(ium).
I also really like Mozilla as an organization and its commitment to open source. (Sure, Google is more committed to open source itself than most large technology corporations, but its methods and motives are often more questionable.)
Currently using the following plugins: AdBlock plus, Stylish, New Tab JumpStart, Tiny Menu, Echofon, GMail Watcher, Google Reader Watcher, and Web Developer.
However, since I’m obsessive about testing my webpages with various browsers, I currently have 11 browsers installed on my primary Ubuntu system: Firefox, Chromium, Opera, Konqueror, rekonq, Midori, Amaya, Arora, Midori, Epiphany and Emacs/W3. I have some older versions of IE running through Wine too. (I use my wife's Windows computer to test things in recent versions of IE and Safari.)