My favorite duo is Natsu and Gray!
These two are always going at it when they are together, but when one of them is in trouble the other will go out of the way to get the other out of trouble, regardless of how much they argue. I like these two so much, because they remind me of me and my sibling when we were younger. We would always argue, but was there for each other by the end of the day. I also like how Natsu is fire and Gray is ice, I guess opposites really do attract! :D
My favorite moment with these two is the Tartaros Arc, when they fought Mard Geer. It was awesome to see these two go all out on Mard Geer after both losing someone that was close to them.