TNG: "Skin of Evil":
If you set the size of the planet to be similar to that of Earth (similar gravity) and find the radius of the fireball it comes out to about 245 km. If you plug in numbers to the stardestroyer.net
nuclear effects calculator until the ground-contact airburst is about 245 km, you get a yield of about 2,321 Gt.
DS9: "Apocalypse Rising":
Assuming an effectively destroyed area of between 200 km and 900 km and six expended torpedoes in a sort of flower-shaped dispersal pattern gives between 90 km and 400 km destroyed per torpedo. Again using the
nuclear effects calculator and plugging in numbers until air blast radius matches those numbers and averaging the results gets about 850 Gt yield per torpedo.
And as for VOY: "Warhead" I found a
site with a database of explosions and a
calculator, and I think either it is not accurate on that scale, or I used it incorrectly because I am getting a rather large 1.98*10^28 J necessary for the event. So I currently have no idea how to quantify this event.
For examples which are on a similar scale, depending on how large the asteroid in TNG: "The Pegasus" is you can get similar figures. If the asteroid is about 90 km on its longer axis and is about 50% solid using 50% vaporized 50% melted gets about 300 Gt per torpedo.