Dreadnought
Dreadnoughts are the siege walkers of the Space Marines. They're piloted by a Space Marine who almost died, but survived only with the Emperor's grace and loads of plot-armor favoritism. If the apothecaries managed to save the life of a marine from wounds that would out-right kill a Space Marine (Which is saying a lot since they can take as much punishment as a heavy battle tank), but can't be restored to battle-capable condition anymore, the marine has the option of being interred into a Dreadnought and pilot it from the confines of his armored sarcophagus.
Dreadnoughts are incredibly tough and their pilots even tougher, they're totally fearless and won't mind that they're gonna die a painful death under the hail of anti-armor fire and the only real way to totally kill a Dread is by blowing it all up or destroying the Sarcophagus. A lot Space Marines in Dreadnoughts are some of the oldest marines in service, some dating before the Horus Heresy, so they got a lot of combat experience, such a Dread is Bjorn the Fell Handed from the Space Wolves.
Tabletop wise, Dreads are an effective all-rounder and are fairly cheap to boot. They can pretty much be good at whatever role they're optimized for, punching tanks and melee troops with CQC weapons or shooting tanks and/or mowing infantry down from a distance with heavy weapons like the assault cannon or lascannon. You can also deep-strike them, ensuring hilarity when you get lucky and they end up where they precisely need to be. With this however, your enemy tends to focus their anti-armor on them, so a footslogging Dread normally won't get far and thus, heavily relies on deep-striking to get the most out of it.
HOW TO MAKE A DREADNOUGHT
1: take 1 badass Space Marine
2: shoot him in the kneecaps and a couple of his lungs
3: leave the dying body on the doorstep of some Adeptus Mechanicus guy
4: ???
5: PROFIT!!! DREADNOUGHT!!!