When it was released? It was the best (ok IE5 on Mac may disagree). Seven years later (when IE7 finally appeared) it was worst.

Initially well, but soon very poorly, because IE6 remained a contemporary browser for an unusually long time.

Distilled down to the core, the fundamental problem with IE6 was the five year chasm where development was completely stalled. When the version was finally bumped in 2006, so much time had elapsed that the solution was unsatisfying for everyone: changes so major that compatibility was problematic, but still insufficient to bring it up to speed with its competitors.

Safari may not be implementing bleeding-edge features at the same rate, but at least the engine is being continually developed and improved.