It is pretty tough as far as the E121 Series of 120 goes. That set is pretty straight-forward with only a small handful of tough variations unlike the Series of 80 which is an absolute nightmare (as any that are trying to put it together have discovered!). The Whitted (Brooklyn) is def among the toughest, and MANY times tougher than the Pittsburgh variation which is common. That being said, I have seen a few showing up lately which is probably because the ones that have sold have done so at a level that may be bringing them out ($80-150 range).
The George Topocer (mispelled) is also from my experience fairly difficult (the correct "Toporcer" is the easier of the two). The Topocer/Toporcer is a fairly new variation discovered within only the last year or so. There are a few other cards that are also difficult but I can't give everything away!
