Definitely legend of Aang. First off, Legend of Korra didn't really feel the same. The Last Airbender had a lot of character development, and everything that happened had meaning for the characters as much as for the plot. Every character faced its own struggles, and it really helped make them feel real. Even the finale wasn't really centered on the action, it had meaning attached to it. All of this, I didn't find in the first 2 seasons of Legend of Korra (I quit at this point). The only character development was Korra's, and it wasn't very pleasant to watch for me. It really feels like they put more effort in the action and the fight scenes than in the actual plot or character development. Even the humor feels weird and out of place, wich is a huge contrast from Toph's blind jokes or Iroh... just being himself.
Also, I know this is less important, but there where a lot of inconsistencies with The Last Airbender, and that kind of thing really bothers me. Like a lot. For instance, 3 yo Korra bending 3 elements (whereas Aang was considered a prodigy for having all 4 of them at 12), the avatar state (established as being a defense mechanism that grants the avatar the power of all his previous incarnations) reduced to a mix of super sayan and Good VS Evil, flying bisons *existing*, portals to the spirit world, bloodbending at day, Bolin inventing lavabending (Roku did), the avatar's journey through the world to experience the philosophies of every nation being absent, Amon's chi blocking, and much more.
In conclusion, I'm not saying Legend of Korra is bad, it's just completely different from The Last Airbender, and it's definitely not the kind of show I can enjoy.