LZII by miles. I've said it for years, the the opening riffs of Whole Lotta Love are among the best opening riffs to any album, ever. LZII was still a lot rawer of an album than the glitz and production that was seen on IV.
On IV, I am so sick and tired of hearing Rock and Roll, Stairway To Heaven, Going to CA, etc. Black Dog isn't so bad but still gets way overplayed. I've also never been a fan of Four Sticks, Battle of Evermore, Going to CA or Misty Mountain Hop. Not bad tunes, but I never got into them. The best song on IV is When the Levee Breaks and strangely enough, its also the song I've heard played (read that as beat to death) on classic rock radio the least.
II is simply awesome!