I dont have the Ironclad model, and am not planning to buy it, so let's keep this to regular dreadnoughts.
I was initially going for a Multi-melta + Powerfist combo (since a turn 1 automatic deepstrike would easily get me into melta range of <12" and make for an easy tank kill), but 7th edition has made one-shotting tanks harder, so whittling down
HPs seems to be the way to go again. Also, because of this:
In this case, I think the Assault Cannon + Powerfist combo seems to be the better choice, given how I could deepstrike behind the tank and shoot into its rear armor. Why powerfist? Because if I deepstrike in the enemy's deployment zone I really dont want to have a S6
AP- walker caught in close combat if I were to kit it out with a Missile Launcher. Those things work better from your side of the table. Should my dreadnought survive, the powerfist would also make for a good triple attack S10 finisher when charging a tank.
I was also thinking about upgrading it to Venerable status, because BS5 instead of 4 can make all the difference in that crucial turn, and its always good to be able to make your opponent re-roll on the vehicle damage table.
What do you think?