I'm interested in LZ (contrary to my avatar) and what are the difficulties for some of the songs? I know that Over the Hills... is considered difficult, Stairway to Heaven is intermediate, but what are some challenging songs that I could learn?

Secondary requirement is that they have decent tabs, but I could always ask one of my friends who loves LZ much more than I do.
I don't think Zep tunes are that hard, but they are challenging.
I think for moderate difficulty - try Heartbreaker.
For something difficult, try Archilles Last Stand.
Getting the timing and feel for the solos is difficult at times, cause Page played with a lot of individuality. The solo section of Heartbreaker is totally off the cuff.
the solos are generally pretty hard to nail, so unless it's something like stairway, i'd just improvise.
rhythm-wise, you just have to listen to the songs to make sure you're getting it right, but they're not all that difficult.
Alright, thanks! I'll probably start with Heartbreaker and work my way up.