What gases can best put out fires effectively? I already know that co2 is one of them.
It's not really a question of what gases put out fire, because CO2 doesn't put out fire, it specifically chokes it out.
Fire is a result of a combustion reaction, which requires oxygen to occur. CO2 takes up the space oxygen normally would, ultimately resulting in fires dying out.
Thus, you shouldn't really be looking to use a gas to put out fire so much as you should be looking for ways to remove oxygen from a fire.
Honestly, though, this would have been a better question for Google than the R&R forum.
To piggyback off what Spoonr said(below me, I think), if you're working with a special chemical fire, nonanomalous gases won't cut it. These chemical fires are typically manmade though, like you're not going to find thermite or chlorine trifluoride in the wild. In fact, with most of these chemical fires, nothing is going to put it out depending on the size of the fire.
Thermite burns at over 2200 K, and with it providing it's own oxidizer, you can't easily stop this fire short of liquid helium, and a lot of it at that.
Chlorine trifluoride is reactive enough that the official documentation on how to handle a fire caused by it is literally to run away. The safety sheet says "Do not attempt to extinguish" because whatever you choose to do, short of a ton of liquid helium (which may not even work), the fire will only get worse. (This chemical is used in rocket fuel, so like Spoonr said, you might want to look into that if that's the kind of fire you're interested in.)
So it really comes down to what kind of fire you're (VoxVillain) talking about. I automatically assumed a typical Class A fire, but neglected to think about the others. Perhaps if you clarified, we'd be able to help more.
If you want, I'd suggest reading up on the Classes of Fires It also comes with typical ways of stopping them.
Keep in mind though that only Class A and B can really be stopped by a normal gas like CO2. I think.
What type of fire? Something standard like burning papers could use anything that displaces oxygen, heavier than air so it doesn’t float away. Halon is a typical example, I assume because it is cheaper than alternatives. Any noble gas heavier than nitrogen should work.
Head over to Wikipedia and look up “fire triangle” for a good overview.
Fancier fires either have oxygen built into the fuel (the iron oxide in thermite), or a different oxidizer that is not based on oxygen. Fluorine definitely makes nasty fires, I think chlorine & gaseous molecules with extra nitrogens bolted on make fire as well. Research rocket fuels to get some more ideas about these chemicals. A lot of these fires, gases wouldn’t work to extinguish because they can’t displace the oxidizer.
The weird possibility would be an anomalous gas that gets reslly really cold when used. Cool the fire to liquid nitrogen temps and that should put out most fires.