1. Rach 2 or Paganini Rhapsody
AND
2. Rachmaninov Op. 3 no. 2 or Op. 23 no. 5
Well, for the second one there is no question...Op. 23 no. 5
The first is a bit tougher to distinguish...for the sake of having an answer, I am going to go with the Paganini Rhapsody. The 2nd is def. Rach's easiest concerto (by no means easy though!).