My Commander, Sek'Kuar
I used to be a devout casual Magic player. I spent most of junior high, high school, and college playing multiplayer games with friends, and while Commander didn't exist yet as a format, I played a lot of Magic with big decks.
These days, I don't get a lot of chances to play casual formats, but I have several Commander decks on MagicOnline and one physical Commander deck.
My commander, as you've probably guessed from the title, is Sek'Kuar, Deathkeeper.
If you're familiar with the deck I've played in Legacy for most of the last year—a Zombie deck based on sacrificing creatures that return from my graveyard over and over—it probably won't surprise you that this is my favorite commander. Like the Edric deck I wrote about last week, the reason I like this deck so much is that it's less concerned with ramping and playing big spells than most Commander decks I build, and is instead based entirely around exploiting synergies, both with the commander and between the cards themselves. The deck doesn't win by playing a single big spell, it wins by adding more and more fitted gears to the synergistic machine it's creating.
Fans of long, drawn-out casual games will want to avoid infinite combos that win on the spot, and will probably want to remove Phyrexian Altar from this deck. An odd choice for the most unfair card, it's just too easy to form an infinite combo with Phyrexian Altar, Nether Traitor, and Sek'Kuar (or any of several other cards in the deck), and the card generally makes the deck extremely explosive.
Players who are looking to win as many games as possible will actually want to go in the exact opposite direction, focus on finding Phyrexian Altar, and add cards like Demonic Tutor, Vampiric Tutor, and Diabolic Intent. I don't play those cards because, while I like the games where I draw Phyrexian Altar because I enjoy the novelty of exploiting such a narrow card, I don't like to have all of my Commander games play out the same way. So I like to minimize my extremely efficient tutors.
As with all Commander decks, this middle ground I've chosen reflects my exact position on how competitive I want my deck to be, which is one of the most personal and customizable aspects of the format.