Dwarfs seem to function best with a solid gunline guarded by large units of Dwarf Warriors, Slayers, and Hammerers. As for having a lot of war machines, the short answer is load up on 'em.
It's hard to say which war machine is "the best" since they all have important purposes. That being said, bolt throwers, cannons, and organ guns are more generally useful than things like the grudge thrower or flame cannon. Bolt throwers have the distinct advantage of not misfiring, though they shoot with the crew's ballistic skill. That's not too bad, considering Dwarfs aren't bad shots-- at long range, which you'll usually be at, you'll hit on 4's. Cannons and the Organ Gun use the artillery dice, but the risk of blowing the gun up by accident does exist. It's not like 40k plasma where it's terribly likely, particularly if you forge particular runes onto the units, but fact remains the risk is there.
The dwarf army I usually play against fields a pair of bolt throwers, a cannon, and an organ gun. It also includes a unit of 20 Dwarf Warriors, a unit of 19 Longbeards lead by a thane with the battle standard , a unit of 18 Hammerers lead by a Dwarf Lord carried by shieldbearers, a unit of 15 Slayers, two units of 10 Thunderers, a gyrocopter, and a running naked Dragon Slayer (just because a dwarf streaking during a battle is sort of amusing...)
The army may look fighty at first glance, but it's really quite shooty, and the big units of warriors and the like are there primarily to protect the gunline. The gyrocopter's main purpose is to march block enemy units, to allow for more time to shoot them. The slayers make a nice speed bump too, since you have to kill every single one of the little bastards before you can continue on.