Firefox. Of all the browsers I keep trying it's the one I keep going back to, especially at work where we run a huge mixture of webapps some as old as 15 years, some only released within the last month or two, Firefox never has any trouble. Our company mandates IE and Chrome, neither of which run the whole spread so users have to keep switching between the two browsers, but I use FF and only have to keep that open.
Mozilla suite, the original before it was "slimmed" to just firefox: It featured a working popup blocker (unlike the current firefox one), had tabs and a unified search/location bar.
Konqueror: Handled a lot of tabs well, could scroll through tabs with the scroll wheel, kept unvisited tabs names a different color, native interface (in KDE). This was porn browsing heaven.
IE: native interface and working single sign on. I've been thinking about the latter lately because my morning routine is to put my password into my desktop, got to the corporate gateway and enter my username and password to start remote desktop (and RSA keys), enter my password again in remote desktop to start my desktop, go to jira (internally hosted) ant type my username and password again (jira prevents saving) to login, enter my username and password in dozens of other places. At least the latter half of these was solved by IE two decades ago and we should have standardized on that somehow.