You can have a single rare choice in armies under 2000 points, or two rare choices for larger armies.
The model: The Dwarf Cannon plastic kit contains the parts to make either an Organ Gun or a Cannon.
The Organ Gun is probably the easiest of the war machines to use and in many cases is one of the most effective. Its range is only 24", but causes a number of hits equal to what you roll on the artillery dice. That means that it is going to average 5 hits every time it fires. If you don't roll a misfire when rolling for the number of hits, you can re-roll the artillery dice but I'd only recommend doing this if you roll a 2 the first time.
This war machine is capable of dealing with anything from skirmishers to knights, and mine has often finished off large monsters like Giants or Greater Daemons that have been wounded by my Bolt Throwers.
The Organ Gun is my first choice when it comes to selecting my rare units.
The model: There is a metal gyrocopter kit. The rotors have a tendency to break, so it is worthwhile pinning them. Drill a small hole in the rotor and the central hub and use a small segment of a paperclip as a reinforcing pin to connect them. Transporting this model can be a pain, so I used rare earth magnets in the hub of the rotor and the top of the rotor shaft so I can remove the rotor between games.
The Gyrocopter gives the Dwarf army something it doesn't normally have, speed. It can be used for attacking enemy war machine crews or enemy wizards. It can be used for march blocking enemy units. It is also ideal for pursuing fleeing enemy units.
Its also worth stating the the Gyrocopter is fun. A typical Dwarf tactic is to sit back and pound your opponent with your artillery and shooting to weaken his attack while waiting for his charge. A Gyrocopter gives you something to do during the movement phase.
As a march blocker the Gyrocopter can move over the opponents units, and position itself on their flank or behind them. Its probably more important to land it in a place to limit your opponent's ability to shoot at you in the following turn than it is to get a good shot with your steam gun. When I've been too aggressive with how I used it, I've had it shot down by all sorts of unlikely things like goblins with throwing axes or units with bows.
I will often use a refused flank deployment (ie most my army on only one half of the board) with the Gyrocopter out on the refused flank by itself. By march blocking the opponents units on the refused flank I've been able to slow down his units to the extent that they can't reach my main lines in a six turn game. At its most successful, I've been able to effectively neutralise units this way that in total were worth up to 3 times the cost of the Gyrocopter.
Note that a gyrocopter that chooses to flee as its charge reaction and then rallies at the start of its next turn, may reform facing any direction and is free to move in the movement phase, but may not shoot or charge. As such its the closest thing Dwarfs have to fast cavalry.
The model: There is a metal flame cannon kit.
The Flame Cannon is probably most effective against targets at a range between 6" and 26". You guess a range of up to 12", add the artillery dice and then place the flame template (which is approximately 8"). Given that the artillery dice will add on average 6" if it doesn't cause a misfire, on average you can expect to be able to hit units as close as 6" to 14" and as far away as 18" to 26".
If the target takes any damage, they will need to take a panic test.