Hello again everybody after what seems an impossibly long hiatus.
in terms of the fluff, indubitably dwarfs have the 'best' artillery, and being able to re-roll misfires and the like is certainly very powerful. Again, in terms of fluff, I'd say the Empire might pip the dwarfs in terms of sheer wrecking power (the impression I got was the Empire can outproduce the dwarfs very considerably), and the Skaven have the highest damage potential.
However, in game terms, I'd actually go for none of these as my top picks. I'd say there are 4 stand-out artillery pieces. All four are for armies which, in the fluff, are meant to be non-shooty (so this is problematic from a fluff point of view), and I do think the rules were poorly written or they were otherwise problematic from a rules mechanics point of view, or (in 3/4 of cases) their rules fundamentally change the way artillery works to avoid the key weaknesses/vulnerabilities of artillery, making them ridiculously good (in my view).
So dwarf artillery does have the advantage of re-rolling misfires (at considerable cost) and stubborn crew, they don't overcome the fundamental weaknesses of being unable to move and fire, being very vulnerable in hand-to-hand combat, and only having 3 wounds.
I'd nominate these four:
1 - the Chaos Hell Cannon. Because it's technically a daemon and a monster, as well as unbreakable (rather than unstable) this means it's basically invulnerable to war machine hunters. It has 5 wounds and a ward save, so it's relatively safe in terms of counter-battery fire (and the chaos dwarf crew count as ablative wounds I believe, as 1/3 of shots from missile weapons hit them). Another major strength lies in hand-to-hand combat - being a fairly decent fighter itself, as well as using its own high toughness (not that of the squishy crew), and being unbreakable and able to thunderstomp means it can beat fast cavalry and absolutely mash infantry. I've seen one used very well as a blocking unit advancing as part of the main battleline as it's very hard indeed to shift in combat. Essentially, this beast of a 'warmachine' avoids many of the key weaknesses of artillery (squishy crew and vulnerability to other war machines, and has a pretty impressive damage potential to boot too. It has the strengths of war machines & monsters, and very little of the weaknesses of either.
2 - The Ogre iron blaster. I'd nominate this because it can move and shoot (this for me is the clincher), has 5 wounds, at T6 it's no more vulnerable to small arms fire than a 'normal' war machine and significantly tougher in hand-to-hand, is pretty powerful in a fight (it's a chariot, after all!), and rolls twice for a bounce and picking the highest - so it's damage potential for shooting is considerably above the average for cannons. Yes, it's 25 points more than a dwarf cannon with forging, but it's much tougher, on average inflicts more damage, and can move and shoot - I know which one I'd choose - every time. the fact that it's a shorter ranged cannon has never mattered in any game I've seen as it can move and shoot and has a longer average bounce, and playing on seriously large tables rarely happens.
3 - The daemon skull cannon. Like the iron blaster it can move and shoot and is a chariot. It also is pretty dangerous in a fight. Having a ward save makes it less vulnerable to counter-battery fire. I don't like facing them. Thematically, if felt like a Warhammer 40,000 kit was shoe-horned into a Warhammer Fantasy list...
4 - the Bretonnian trebuchet. This piece has its power through rules changes between editions. In 6th ed, when the book was published, the trebuchet was pretty balanced. Fast forward to 8th ed, stone throwers are vastly more accurate (no more range guessing),and all partial hits from templates auto-hit. Also, most stone throwers now have S3 - whereas the trebuchet was still S5. The devastation this thing could do to T3 infantry was extreme, particularly as it was so cheap. Also, having 4 wounds made it less vulnerable to counter-batter fire (on average one wounding cannon hit won't kill it, when it would kill most war machines).
The Bretonnian trebuchet was least problematic in game as the Bretonnian book as a whole was fairly weak by 8th ed, and traditional tactics against the trebuchet still worked (war machine hunters, getting into hand-to-hand, etc). Also, this one rankled less in game as the trebuchet was so powerful because of rules changes between editions, not because of (in my view) questionable rules writing for 8th edition army books.
Most of the niggles for the first three on the list could be easily addressed. Making the Hell Cannon unstable or just less dangerous in a fight would redress the issues (it's the same for daemon princes in Warriors of Chaos armies - I've no idea why these daemon princes could take common magic items and were unbreakable rather than unstable, whereas neither applied to daemon princes in daemon armies), and making the Iron Blaster & Skull Cannon into move-or-shoot weapons would address a lot of the niggles (also, making the Iron Blaster T5, not T6, would have helped a lot - I've no idea why it was T6).
Anyway, I'd respectfully say that the dwarfs don't have the 'best' artillery in Warhammer 8th, at least not point-for-point. The most fearsome option available to dwarfs is probably an organ gun with forging and accuracy - but at 170 points it's still very vulnerable to shooting and in hand-to-hand, and cannot move-and-shoot (so, unlike the Iron Blaster or Skull Cannon, couldn't hide behind a house and then pop out to shoot in Turn 1).
However, dwarfs did have the option of going gunline, which none of the armies to which the four 'best' war machines on my list could do. But I'd strongly say that point-for-point the dwarf artillery was significantly less good than any of the four on my list.