In my state, my favorite ant would have to be Pogonomyrmex badius. Not only are they morphologically amazing to look at, being one of only two polymorphic Pogonomyrmex species, but they have an extremely interesting ecology, only being found in areas that burn frequently, like Longleaf Pine Savannas and scrublands. There's still a lot we don't know about them as well, like why they move from their intricately-constructed nests to build an identical one once a year or why they gather bits of charcoal to place around their nest entrances, and cease to forage when the charcoal is removed until it is replaced. Lots of things to love about these gals.
My least favorite ant in my state is either Linepithema humile or Brachyponera chinensis. It's hard to decide which one is worse. On the one hand, Linepithema form massive colonies spanning entire continents and are extremely competitive, and on the other, Brachyponera chinensis actually directly competes with Linepithema, kills off native termites, and forces out native forest ants like Aphaenogaster. I don't mind Solenopsis invicta too much, as they don't compete much with the native ant fauna (the one exception being our two native fire ants), but both Linepithema and Brachyponera can go suck a jellyfish for all I care.
My favorite ant in general would have to be either Strumigenys agra or Strumigenys dbb072 (a morphotaxon collected by Andrea Lucky and identified by Douglas Booher). Strumigenys agra is a chocolate brown, giant dacetine from Madagascar. Reaching lengths of 5.4 mm, it is one of the largest Strumigenys. It has long, upwards-pointing propodeal spines, long appendages, and a greatly elongated head tipped with trap-jaw mandibles. On the other hand, Strumigenys dbb072 is a New Guinean species with very little information recorded on it. It's bicolored yellow and dark brown, with a strongly shining gaster. It is also extremely leggy, and sports long, gently curving ventrally propodeal spines. Its appendages are ridiculously elongated, to the point where the hind limbs do not even show up in the AntWeb images because of how long they are.
My least favorite ant in general is Pheidole megacephala. It's taken over entire island, exterminating natives as it goes, has completely overrun mainland areas, and is just all-around bad news for anything that gets in the way of its path of destruction.