Marneus Augustus Calgar, Chapter Master of the Ultramarines
Calgar is the superhero character of Codex: Space Marines. He has an extra wound and attack compared to a standard Chapter Master, and wields a pair of power fists along with an AP2 storm bolter. To make him nastier, he can re-roll all his failed to wound rolls in shooting and assault. To top it off, he’s Eternal Warrior. He comes with power armor, but it’s absolutely worth upgrading him to Terminator armor. As an individual, Calgar can both take it and dish it out.
Calgar also benefits your entire army. Due to his God of War rule, every unit in his army with Combat Tactics can choose whether to pass or fail any morale check. This means your units will never fall back when you don’t want them to, which is a pretty massive ability.
Finally, Calgar allows you to take up to three Honour Guard squads in your army rather than the single squad a generic Chapter Master allows. Since Honour Guard squads are pretty one-dimensional and expensive in points, I don’t see much reason to take more than a single squad. However, if you have a reason to take multiple squads of Honour Guard, Calgar lets you do it.
For all the damage that he can personally inflict and what he brings to the army, Calgar costs more points than a Land Raider when taken with his obligatory Terminator Armor. For this expense, you’ll have to carefully consider if Calgar is worth it.