Shostakovich 1 is not easy. #2 is not hard.
Anyhow, looking for the easiest piano concerto is weird. Piano concertos are usually big time show-off pieces.
From what I have heard here are the most popular concertos for young students:
Grieg, Schumann, Beethoven 3, Mendelssohn (g minor), some Kabalevsky, Shostakovich 2, Saint-Saens 2. They all sound harder than they are...