Board Thread:Questions and Answers/@comment-25570259-20141023000007/@comment-4651179-20141105082630

I'm not an expert of Thanos, but I found a page (here) with a list of "The Greates Thanos Stories Ever Told." It should be helpful.

I would also add Infinity. It was a 2013 event which tied-in with Jonathan Hickman's Avengers and New Avengers. If you're already reading those two titles, it could be a good oportunity to read Infinity, as they compliment it a lot. Infinity is also, so far, my favorite Marvel event.

And I'm not an X-Men X-pert, so I can't really say which stories are the best, sorry.