The Dardanelles Bombard is the one gun that I would probably sleep with. Although Pumhart von Steyr, Dulle Griet and Schwerer Gustav (not medieval) are close.
The Sick Man of Europe has some p sick artillery.
EDIT THAT'S APPARRENTLY THE TSAR CANNON BECAUSE GOOGLE IS USELESS
Thanks for that explanation. The sniper looking dude's weapon is called the Hochland Long Rifle, so the word is plainly used in-setting. Meaning that Games Workshop just dunno what the hell they're doing, as per usual. I'm calling them riflemen and nobody can stop me
More bombards, cannon and superguns for personal inspiration - all of my artillery will be named.

Mons Meg, French-crafted gift to James II of Scotland.

Queen Elizabeth's Rocket Pistol, with a range of over 2000 yards. This is weird.

Dulle Griet (Mad Greta), Belgium's supergun.

Pumhart von Steyr, Habsburg Austria. The largest-known wrought-iron bombard by caliber.

THIS monster is apparently the Dardanelles Gun, which was used to breach the walls of Constantinople- and also, 500 years later, it was used to shoot at British warships with equal success.

Schwerer Gustav, Nazi Germany, WW2. The railway gun nobody ever knew was needed, and it also wasn't.