Wikipedia surprisingly does not contain a good list. Under philosophy of mathematics it is rather vacant too. A graduate level list is here, but I was curious more about the most influential papers. Manually one could search here. But all in all, the database out there is rather barren.
Edit: For some reason when I was typing the title it did not suggest me this math.SE question. (will delete if duplicate).
All these lists are bound to be incomplete and more subjective than one would like, but a truly excellent selection of landmark papers can be found in the book "Mathematical logic in the 20th century", edited by Gerald Sacks, World Scientific, 2003.
Here is the table of contents:
(Full bibliographical details for each paper can be found in the book, of course.)