My favourite species of ant has to be Eciton burchellii, the Eciton army ant. I love how this species is an incredible super organism that pretty much dominates it's eco system, I doubt even large mammals would want to tangle with them. But what I love about them the most is the fact that they're completely nomadic, having no permanent static nest but instead forming their nest from their own bodies into a 'bivouac' and relocating every few days to exploit wider feeding locations. An incredible adaptation.
Fire ants have taken that concept a step further in their adaptation to an environment that is susceptible to frequent flooding. I remember seeing this in a documentary once and was completely amazed. Apparently the ants are completely waterproof enabling them to make a floating nest until the flood subsides and they can find a new nest site.