This is just me but I always want a flying type on my team so I can fly from city to city but if you don't mind training a pokemon just before the elite four Salamence is a great choice. However, if you don't want to train a pokemon late game, here are some other options: Flygon, Skarmory, Crobat, or Xatu. Out of all those, I would choose Xatu, because you don't have a psychic pokemon on your team yet. One more tip, four of your pokemon have a 4x weakness to something, Swampert, (Grass) Aggron, (Fighting/Ground), Breloom, (Flying), and Canerupt (Water) be sure to be aware of these weaknesses