oh boy, some ancient stuff right there!
Honestly a lot depends on what you consider op.
If we are talking "speedclear meta", I think the classic pouncesin with craft was faster (that was after this cotw video, back in that version you coudln't craft pounce naginata iirc). Just more efficient, procs that actually do something, instead of looking nice like glacial nova.
There was patch where bowazon was "good". It was version where bow skill originally called dragonforce was at its peak, absurd range and hit rate, also weapon modifier. Long story short - it was very strong. Then mix it with demonstorm unique bow, and proc was also extremely strong. This character might not be remembered as pouncesins and such, but considering how cheap and simple it was, the clear speed was ridiculous. And it was homing attack that clears everything in front of you, no need to aim anything, just smooth 100% clear. Unfortunately I didnt make video of it, it would be fun to watch today.
But then you have this aspect of backtracking for items. It is true that realistically a character that is too fast isnt good because there is no time to pick drops. Same goes for characters that heavily rely on life on kill and proc chain to keep them going. If you stop you lose momentum.
If you take more recent times, for example leveling past 120 in cows, unholy pala with apocalypse. This is perfect example, shift+click, screen dies, no procs, no lag, just clean clear, you don't need to aim anything, you never get stuck, you can pick drops pretty easily. Ofc cows arent much of a challenge, but it is so much more comfortoble to run with this, than for example a spearzon with pounce, even though speeds are comparable.
And if you allow sligtly lower killing speeds, the old original lazydin was pretty cool character. You had gigantic aoe (more then screen) weapon damage aura that triggered procs and poison + slow. You didn't have to do anything, just walk/teleport, picking up drops while watching tv show in another window
And all things considered kill speed was good.
What I also remember, from even more ancient times, around median 2008. There used to be a runeword with 25% chance to cast fortress on kill, back when fortress towers aimed multiple targets. Could be one of more op things ever, it was ridiculous (quickly nerfed). A rw that gave barb retaliate oskill (when it had nearly unlimited range) was quite good too.. Or the super old stuff, with permanent unholy armor, but that wasn't really all that useful, more for a show.
When it comes to character op vs heroic bosses, I think it might be just a sorc with starfire oskill, and poison. There was version where you could just break their poison immunity, and apply millions of poison damage via spell like diseased cattle. It was also back when poison spells were extremly overtuned, they were close to damage cap per frame. With char like that you could kill astrogha bosspack in one attack