My personal top 5 are as follows...
5: Erza vs. the 100 monsters (Grand Magic Games Ark)
4: Natsu, Gajeel, and Wendy vs. Faust (Edolas Ark)
3: Erza vs. Azuma (Tenrou Island Ark)
2: Natsu and Gajeel vs. Sting and Rouge (Grand Magic Games Ark)
1: Natsu vs. Jellal (Tower of Heaven Ark)
I kind of find it weird how the best fights involve either Erza or Natsu.