Red and Black decks, so named Rakdos after that guild's color combination, were quite strong in the days of Return to Ravnica Standard. the leader himself and the namesake of the guild Rakdos himself didn't see too much competitive play. His battle-cruiser type ability seemed best reserved for an EDH deck built specifically around him. indeed, he would become a very powerful commander as time went on for a variety of reasons.
But in Standard, Rakdos could serve as a more efficient one-of creature than many people realized. First of all, a 6/6 Flyer with Trample for only 4 mana is downright ridiculous. His only real downside is that you can't cast him unless an opponent has lost life that turn. in commander, there are about 2 million ways for that to happen. In Standard, there were obviously not so many. But it was most certainly doable given how aggressive Rakdos decks were, and still are really. on raw power alone, this guy was very playable.
But on top of that, its his secondary ability which is extremely interesting. For each point of damage dealt to opponents that turn, Rakdos makes your creature spells that you cast cost X fewer colorless mana to cast. It's this ability which makes this card a truly good boss monster in a Rakdos deck. It would also be the key to his power as a Commander.
Being able to make creature spells cost less is always a good thing. Considering how often it is that a Rakdos deck causes damage to opponents, you're likely going to be casting a LOT of cheap monsters. In commander, considering his ability reads OPPONENTS, this allows for even more deadly tactics. this is because effects that deal damage to all opponents or players become a lot more relevant.
However, there were practical synergies to take advantage of in his Standard heydey. Using a simple commonly played burn spell like Searing Spear would reduce your creatures casting costs for the rest of the turn be reduced by 3 colorless mana. Then, you could find yourself dropping a Thundermaw Hellkite for merely two red mana. This is slightly absurd.
Rakdos, Lord of Riots also made other big dragons of the time like Moonveil Dragon and Balefire Dragon suddenly far less intimidating to cast. In the right situation, you could drop a lot of big scary stuff in one fell swoop. Heck, you could potentially make Griselbrand cost only 4 black. I don't recall this interaction being taken advantage of in Standard, but it would have been a good one. Sadly, it doesn't work in Commander, as Griselbrand is banned in that format for many reasons.
Rakdos makes a very good Commander when used alongside great cards like Olivia Voldaren (another fine Rakdos card) and other cards like Chandra of the Firebrand that can consistently ping players.
On the other hand, his cheap casting cost makes playing two or three in a Constructed deck very manageable. However, no Standard decks really ever wanted that many copies and other constructed formats found him a bit too situational to brew around.
While we were never going to see any top-tier lists running Rakdos, that doesn't mean he wasn't a good card at the time. he actually made some appearances at local Friday Night Magic tournaments for a bit. In the long run, he's become an excellent Commander. he's also a very useful member of other Commander decks looking to swarm the board quickly and efficiently with big bad boy monsters.
Don't overlook him, cause a 6/6 trampling flyer alone can be enough to wreck your day. Plus, he can bring a lot of friends along with him, too!