Hm, you find "The Long Road To Lemming High" hard? I guess you're referring to the version from the Fewer Builders challenge then(or the 100% version if I ever released that), as the regular version is pretty easy.
Anyway, if you want something hard from me, check the new topic I just made("Heh, I love it when this happens...").
In my opinion, it's either When Hell Freezes Over (from my Windows remake) or To The End! (from Lemmings Plus 3). Oblivion (from Ultimate Challenge 3) and its variants(mainly Temple of Oblivion, the others are kinda different and easier) would also be contenders, except that by now /everyone/ knows the trick, I think.