I say Aang.
Korra was born after Aang brought peace and prosperity (overall seen) to the world lasting several decades and as a result Korra had a very easy, comforting life till the start of LoK in which she mastered three elements without much effort. Aang on the other hand was much younger, only knew one element, had to learn all 3 other elements in a single year and this after having been trapped inside an iceberg for a hundred years. To top it off during all that time the world was at a constant state of war. Almost everyone alive has known nothing else than being at war and almost everyone Aang knew had died, including his entire culture.
For Aang this was just a horror scenario, while Korra had nearly everything going for her. I mean even Bolin said it at one point: "I wouldn't say you had it rough". Korra could profit enormously from Aang's accomplishments, but Aang had to do almost everything from scratch.
Finally the way how Aang was perceived by people was mixed. For the Fire Nation he was an enemy and many people also lost faith in the Avatar for leaving the world as it was for a near century. Aang had to 'redeem' himself, while Korra could continue on the image the world obtained of the Avatar due to Aang's accomplishments: that of a hero. Aang always had to be very cautious of mentioning him being the Avatar, Korra couldn't wait screaming it off the rooftops.
None of the problems Korra had to deal with in seasons 1 and 3 (so far) exceed those of the ones Aang had to deal with. Most of the events of season 2 also don't exceed Aang's problems, with the exception of one:
Korra had to deal with the Dark Avatar and that's a far more bigger issue than all Aang's problems combined as no matter how horrifying Aang's world could have ended up with if Aang hadn't been there, it would still be peanuts compared to 10 000 years of complete darkness with a world ruled by a Dark Avatar and the end of a 10 000 year line of the Light Avatars. Though IMO I think Aang would have handled it better than Korra, though saying that is probably a bit unfair as that's how the plot works.