Kingprotea is the strongest servant. She doesn't just have one completely broken ability, but several that combine together that make her a "universe-scale" threat.
She is the physically strongest servant, while also having literally infinite power that grows over time, which is only kept in check thanks to a "curse" bestowed upon her by her creator that forcibly resets her power after a certain amount of time. However, thanks to also having the rare and completely broken ability to modify her own Saint Graph, she is slowly eroding that curse and growing stronger between each cycle of resets.
She is strong enough to punch through magic and concepts, is immune to mental control or influences, and is also completely independent of her master and able to infinitely supply her own energy.
Nasu even said that if she were ever summoned into a holy grail war, she would certainly win, but would also have a 99% chance of killing her master, because her only real weakness is that she is mentally unstable, and there's only a 1% chance that a master that summons her would be truly compatible with her.
She is also adorable.