In addition to the Novalith, Advent can use their supercanon to drop a world with low allegiance to 0, effectively killing it. It has been months since I experimented with it, but I played a game a long time ago where I used multiple cannon blasts on higher allegiance worlds, and got them to hit 0. I haven't experimented with this in ages, so I don't really have any useful advice as far as how much allegiance one canon blast can shift, but I know it is possible.
As far as ships go, the three best for smoking a planet are:
Advent Revelation @ level 6 w/ Provoke Hysteria
Vasari Evacuator @ level 6 w/ Drain Planet
TEC Marza w/ 3 levels of Raze Planet
Vasari Desolator w/ 3 levels of Deploy Siege Platform
The Evacuator has a long cool down for Drain Planet...but does about 1350 in one use, so it will just about smoke an un-upgraded planet, then finish it off in 30 seconds or so of additional bombardment. I don't play Advent much, but the Revelation does a huge chunk of damage as well, and I don't remember the cool down being as large.
The Marza special ability doesn't do as much damage in one burst, but it cycles fairly quickly. It chews up planets and spits em out pretty fast. The Marza might be fastest at upgraded worlds. It would be interesting to time it versus a Revelation.
The Desolator is not quite as flashy as the above ships, lacking any sort of burst damage, but it isn't a slouch in the planet melting department either. The Siege Platforms provide a steady damage increase.