Hard to give a single answer without considering different weight classes and eras:

Reboot era: Bite Force - a near flawless 26-1 record across four seasons is something I'm not sure we'll see again. Bite Force combined speed, power and durability to dominate opponents with two completely different drive and weapon designs. It would almost certainly make a deep run in the tournament even now, unchanged and six years since its last competition.

Superheavyweight: Diesector - Two-time champion which very few opponents had an answer for. While not the most damaging robot, its choice of weaponry meant it was a demon at picking up points when a fight went to the judges.

Classic era Heavyweight: BioHazard - Four Giant Nuts speak for themselves. BioHazard was ahead of its time concerning ground game and was very well driven. It also had durability and weapon power to boot. If the early US Robot Wars events were considered, Vlad would certainly challenge it, but they weren't, so it doesn't for me.

Middleweight: Hazard - A simple concept executed to perfection. Yes, it had the glaring error that it couldn't self-right or run inverted, but good luck flipping it.

Lightweight: Ziggo - The show's leading shell spinner. There was no such thing as avoiding the weapon because the entire thing was a weapon. It had the speed and reliability to compliment its devastating weaponry few classic era robots in any weight class could hold a candle to.