Hi,
So much depends on your level of experience and knowledge of any of those schools. From my perspective, Lehigh Valley rifles would be the most challenging because they can turn out very awkward looking and rather uncomfortable to shoot when shaped poorly and the nuances for getting them right are in some respects obvious and in others, subtle. For me "Golden Age" period Lancaster rifles are the easiest and least subtly nuanced. However, none of this applies to decoration because that is much more individualistic even within schools. To my mind the works of the 2 Isaacs, Haines and Berlin, are at the top of the heap for difficult carving decoration. John Fleeger comes to my mind with respect to challenging engraving. For easiest to decorate? Just about any American muzzleloading rifle after 1830 or so.
With respect to all guns, British and French fowlers, rifles, muskets, and pistols from the 18th and early 19th centuries are very difficult to get right and do justice to. That stems from a lack of knowledge but also a lack of correct parts. It is much easier to find correct parts for an 18th century American long rifle than it is for an 18th century British or French fowler.
dave