Um, I'm a bit confuzzled but...
My favorite concerto (and my favorite Rach) is his 2nd. Although I think that the peak of his output was around the time of his Third concerto, first sonata, and etudes-tableaux.
Seeing how the first Rachmaninoff I ever heard was his first sonata, I'd have to say that I prefer that one to his second. NO ONE ever listens to - or plays for that matter - his first sonata, which makes me veery verrry sad. It's like all of Rachmaninoff's motifs in a 45 minute long masterwork. Amazing.
And as for why I like his second concerto..... hm.... well, It always stood out to me more than the others, and it represents a great deal to me. The melodies are so rich, and get stuck in your head worse than cheap boyband music you hear on the radio.
IDK. Rachmaninoff was king of the piano concerti (of the standard rep, in my humble opinion...) and the only other concerto that would come close to any of these colossal works would be Prokofiev's 2nd. Other than that, no others can even come close to the emotional intensity of Rachmaninoff's concerti.
But that of course, is just my opinion.