It's varied through the editions. 3rd edition Nightbringer was a natural disaster -- I remember feeling bad when I fielded him because there was very little that could slow him down. 5th scaled him back a lot, although in 8th he wasn't terrible because you could hide him in a Scarab Swarm and deliver him into the face of your enemy where he was most effective.
9th edition, though, Nightbringer is a goddamn rape machine. I would say Void Dragon is right behind him in terms of ass-crushing, but Super Goth Buttfucker is going to be a prime pick even with his godawful 350 point cost.
EDIT: Sigh, of course you meant Monoliths... although a lot of things I just said about NB apply to the 'liths as well. 3E Monoliths were walls of fuck-you, only stopped by massed heavy tank fire (AV 14 on all sides and ignored any armor-reduction effects). In 8th they weren't bad, but their range was somewhat anemic compared to other factions' heavy vehicles.
9E definitely buffs them with the death ray option and the range buff to the particle whip. With 24 wounds, Living Metal, and a Canoptek Spyder, they may be big targets but they'll be hard ones.