Robinson is the winner with a 90-95 unbeaten run, 2nd Chavez who got to around 87-0, Monzon 3rd whose undefeated streak lasted something like 82 fights.
I would give it to Mayweather, but he lost that first fight to Castillo. Marciano... retires at 49-0 after beating everyone in the HW class. I don't know how much more impressive you can get.
Benn's was the most impressive simply from a visual point of view, 22 (or so) fights unbeaten with 90% of them ending with a stunning knockout
Armstrong and Robinson had very impressive unbeaten streaks from memory. As far as quality wins and achievements I would think they are hard to beat. Monzon had a fair old run as well. Jimmy Wilde was also 94 fights unbeaten, with lesser opposition than the above.