I used IE briefly in 2000 and 2001, before Mozilla (0.9) became usable enough and Phoenix came out; back then (IE 5.5 to IE 6.0) it definitely was the best browser, by far.
It was the most forgiving browser, with a quirks mode that handled broken code like a champ. But Opera was the best browser, and if your code rendered correctly in it, it would probably render correctly everywhere else (except maybe Netscape 4, an abomination that took too long to die).
Opera was also commercial. At some point there was a free adware version, but I think that was later than 2001. After IE, most people stopped paying for web browsers.