Dreadnought
The Dreadnought has generally fallen out of favor, but I think that it’s undeserved. It’s fairly durable for its price, and can act either as a close range threat with an assault cannon or multi-melta and a power fist or it can act as a fire platform with an extremely cost effective pair of twin-linked autocannons or the more expensive but harder hitting twin-linked lascannon and missile launcher. It’s particularly important because this is the most efficient long ranged firepower you can get outside of Heavy Support.
Given the reduced presence of long range anti-tank weapons in most armies, the fact that power fists seem to be exceedingly rare, and the increase in vehicle durability in 7th Edition, the Dreadnought is much more difficult to destroy than its reputation implies. Either take a long range armament and deploy it in cover for a durable weapons platform or keep the power fist and use a Drop Pod to threaten the enemy Turn 1. The multi-melta and assault cannon are the obvious choices, but you can also take a twin-linked heavy flamer and replace the storm bolter with a heavy flamer. This format will tear through any squad with a 4+ save or worse, and is guaranteed to earn First Blood in many match-ups. Deployed carefully, a Dreadnought in the enemy lines on Turn 1 can be extremely difficult to deal with and can buy a lot of breathing room for the rest of your army. This is almost a requirement for a Drop Pod army.
The last question when fielding a Dreadnought is whether to upgrade it to Venerable. This gives the Dreadnought WS and BS 5 and allows you to force your opponent to re-roll any vehicle damage rolls, meaning you can save it from a lucky Explodes result. It won’t save it for too long as it can still be wrecked by losing hull points, but it will make sure that your opponent has to damage it three times to kill it. Honestly, I think the upgrade is most useful for the boost in BS. If you’re relying on a single shot weapon like the multi-melta or missile launcher, it’s an excellent upgrade to take. It’s not a bad upgrade in other circumstances if you have the spare points.
Ironclad Dreadnought
I mentioned the close-assault Drop Pod Dreadnought above. The Ironclad is that, turned up to eleven. With AV13 on the front and side, it is extremely difficult to kill, especially in assault. It carries a power fist and either a chain fist or a seismic hammer (essentially a AP1, concussive power fist), so has an extra attack and extra damage against vehicles compared to the standard Dreadnought. It comes with a storm bolter and meltagun, and both can be replaced by heavy flamers. If you want some longer reach, it can be armed with two hunter-killer missiles. Ironclad assault launchers basically give it assault and defensive grenades, meaning it doesn’t suffer the initiative penalty for assaulting into terrain and enemies don’t get a bonus attack for charging it. It’s cute and it’s pretty cheap, but few enemies will be able to hurt this beast in assault. Finally, you can replace the power fist with a hurricane bolter. This is insane and you should never do it.
The Ironclad Dreadnought fills one role in the battle: it drops into the enemy’s face, destroys a unit (either a vehicle with hunter-killer missiles and a meltagun or infantry with two heavy flamers), dares the enemy to destroy it, and then charges in and demolishes the enemy until they find something that can deal with an AV13 walker in assault. You can deliver the Ironclad with either a Drop Pod or a Stormraven. The Drop Pod is cheaper and will deploy on Turn 1, but the Stormraven can deliver the Dreadnought directly into assault. Either option is workable, though I prefer the cheaper and more reliable Drop Pod.