I really love both albums, but I think IV wins out due to the sheer strength of the songs - seven of the eight songs are classics in my view (Four Sticks is the only one that falls short of that exalted status, but I still like it a lot). Black Dog, Stairway To Heaven and When The Levee Breaks are my three absolute favourites and would all be in my top 10 Zep songs. Led Zeppelin II also has several standout songs, with Heartbreaker being my personal favourite and no real weak points - I even like the drum solo in Moby Dick - but I think IV as a complete album is just a bit more impressive.