Crassus Armored Assault Transport
The CRASSUS ARMOURED ASSAULT TRANSPORT is one of the heaviest armored transports available to the Imperial Guard. Though it does not carry as many soldiers as the Gorgon (with which it seems to share a chassis, as well as the Macharius), the CRASSUS ARMOURED ASSAULT TRANSPORT is fully enclosed and armored all around.
Plus the Gorgon suffers for simply not being the CRASSUS ARMOURED ASSAULT TRANSPORT.
Dominus Armoured Siege Bombard
Another variant of the CRASSUS ARMOURED ASSAULT TRANSPORT the DOMINUS ARMOURED SIEGE BOMBARD replaces the troop transport capability with a massive Dominus Triple Bombard. This turns it into a dedicated artillery piece that can rapid fire due to its automated weapon system and the fact that it carries not one, not two but THREE enormous mortars, meaning that it can devastate entire blobs of armies in a single phase. It's the Wyvern's bigger, angrier brother with a prison record.
In practice the Dominus throws 10d6 S10 ap-3 shots with indirect fire when stationary, easily wiping any unit foolish enough to be larger than 5 models. S10 Ap3 is also a serious threat to any armour short of a titan, and the weight of shots pretty much guarantees an overkill of single model targets. Think of it as an entire battery of Basilisks in one vehicle, except for less points, with better armor, no Open-Topped bullshit, and a pair of sponsons to defend itself.
One final point of delight in this things favor comes from how overwatch works - not only does it get to fire everything it has at anything that charges it (which will frequently paste dreadnoughts even hitting on 6s) but can simply drive out of combat and keep shooting thanks to its Steel Behemoth ability.
Praetor Armoured Assault Launcher
Much as the Chimera was modified into the Manticore Rocket Launcher by adding a missile launcher to the top, the PRAETOR ARMOURED ASSAULT LAUNCHER mounts a massive Praetor missile launcher instead of a transport bay. It can launch Foehammer missiles for cracking armor, Firestorm missiles for blasting large infantry formations, or Pilum (Latin, "Javelin") missiles for anti-air defense.
Considering that the Praetor launcher has no limit on ammunition (during the game, anyway), a comparison to the Rhino-based Whirlwind might be more suitable.