I gotta go with the robots on this one.
Especially the older stuff like Mazinger and Getter Robo, I mean this stuff was so over the top and ridiculous that when Gundam came along it was the serious and realistic change in tone for giant robots as a genre. Speaking of gundam, the Crossbones series, space pirate giant robots.
The megadeus from Big O are all pretty great additions as well.
And what about stuff from games? The Metal Gears, the mechs from the various Xeno games, Orbital Frames from Z.O.E, Chrome Hounds or even better the secret weapons of the three countries, Armored Cores, and the various titans of Warhammer 40k.
A good chunk of them are designed strictly for fighting giant monsters from the beyond and they do it excessively well.