My favorite thing about Commander is how you're encouraged to consider your opponents' fun. More often than not they are friends sitting across from you at your LGS or dining room table. The creative nature of the format can foster a unique and ever-changing environment every time you play, and this alone can lead to some incredibly interesting house rules.
House rules are fun, unofficial additions to the basic rules of Magic that can be adopted by any playgroup after a discussion. They can impact everything from turn order to how the game is played, and twist Commander in exciting new directions.
This house rule was the first one I encountered. In fact, I thought it was standard practice because it was introduced to me when I first started playing. Each of the players in your game shuffles their deck and then cuts their deck at any spot. Wherever they cut, they reveal the bottom card. The player who reveals the card with the highest mana value will go first.
This house rule will always have a special place in my heart, though it can be a bit unfair at times. Decks that run high cost spells will have a greater chance at starting the game off first, even if the deck is shuffled thoroughly. While this house rule can give players an advantage, it still is a blast to implement at tables and can even turn into a game in itself.
While "Highest Mana Value" was the first way I learned to determine who starts the game, my current favorite way is Poker Roll. Players take five d6 and roll them. Whoever rolls the best "hand" goes first. The goal is to get the highest number of matching rolls or roll a specific combination like a full house, a straight, or a five of a kind.
As someone who wasn't familiar with poker, it was a bit difficult to learn the rules of this house rule, however, it has quickly become a favorite of mine that has somewhat transferred to my own stream.
This house rule is new to me but is still one I find to be a fun game on its own. This house rule is known as speed commander!
How many times have you wanted a quick game but ended up playing over three hours? That is one of Commander's fatal flaws. However, with speed Commander, the game changes altogether! You of course still have a 99 card deck, a commander, and 40 life. Instead, how you play the game changes. Rather than drawing one card each turn, you draw two. You can play two lands each turn as opposed to one. This makes the game much quicker than normal, which can help you get home quicker or help you play as many games as you can while you're at your LGS. Maximum value means maximum fun in this instance!
This one is always a bit of a fan favorite, falling under the infamous Rule 0 conversation. Too often have Commander players' hopes been dashed when illustrious pieces such as Sanctum of All or Elbrus, the Binding Blade turn out to be illegal commanders. Sure, they're legendary, but they aren't creatures. They can't be your commander. However, at the right table with the right conversations, they can be!
Legendary enchantments, artifacts, and yes, even Planeswalkers can become your commander with this unique house rule. Now this particular rule tends to bend the overall rules of the game a bit more than our previous house rules, so it is imperative that you have a conversation with your playgroup about this before bringing a deck with a noncreature legendary to the table. Still, how cool is it to win a game with The Meathook Massacre as your commander?!
Our #1 house rule is very much in the spirit of the format. In fact, it is one that I apply to my games with friends as well as my games on stream.
Unlimited free mulligans is not always the norm, but it is a house rule I find that helps maintain a healthy play environment. The game is not fun unless everyone has an opportunity to play, and seeing your opponents get mana screwed or draw nothing playable in their first hand is not fun at all. With infinite free mulligans, each player has an opportunity to get a hand that is playable (which is vastly different from sculpting a hand). It's because of this house rule that I've been able to have so much fun playing on stream and playing with friends at my card shop.
This house rule sits at #1 because it considers the enjoyment of your opponents as well as yourself.