I gave brawl a shot and managed to convince a few of my commander friends to join for a while. It was pretty unpopular. I remember thinking "why don't we just play commander?" We did have some standard players join in, but they didn't really have a sense of what makes a good multiplayer deck, so their card choices were wonky. And if you weren't playing green, mana ramp was not something your decks would have. I kept my deck until rotation. I think I was the last person in the city with a brawl deck.
Oathbreaker on the other hand went really well. I had
Xenagos the Reveler +
Genesis Wave and
Jace, Wielder of Mysteries +
Paradigm Shift. Lots of fun. Myself and a few others pretty quickly figured out the format. There are two type of signature spells that make a successful deck; either a spell that ends the game when you cast it, or a spell that can be discounted so you can cast it multiple times.
I've also considered Conquest. It's made by a group of people who didn't like the way the commander rules committee was running things. They've tailored Conquest's rules and banlist so that aggro decks are just as viable a strategy as combo decks.
And one thing that I really like with both oathbreaker and conquest; the creators listen to community feedback, and are willing to make changes based on it. That is something that's been lacking from commander for a long time now, but we are starting to see changes in that direction.