Most definately Alexander Johnson, he can step in right away and will still improve almost as much as a guy like Shawne or Hilton.
I don't know why you would want Shelden williams, for one, he got OWNED repeatedly last season by Alex Johnson, and is only 6'7.5". Dude is a shorter version of Etan Thomas, not a good pick at ALL.
Check him out, he aint no pudgy boy waiting to go through puberty anymore, he is a monster, and at 23 has an NBA body, skills, a 40" vert, a mean streak, and though his #'s were unimpressive, he was doubled on EVERY posession and always fronted on defense so couldn't get many weakside blocks. His game will translate to the NBA real well, just take a look: