Which countries are the most complex to collect?

Indian Feudatory/Princely States (aka the Uglies)

About 40 different states with issues from 1860s through 1948. Many are extraordinarily primitive on many kinds of papers. Some printed in watercolor based inks that you can't soak or the image just disappears. Many non English scripts. And to top it off various kinds of counterfeits (some done by post office officials), copies, reprints (some official) and fantasies. Even one current eBay seller producing fake cancels left and right on fairly rare but genuine stamps. Go figure.

One of my favorite states is Jammu & Kashmir. Below is a pair of Half Anna stamps from there cancelled with the brick red obliterator from Srinigar -- SG91 from 1866-77. Watercolor.



Below is another SG91 Half Anna Kashmir blue mint, no gum (as issued). You can see the legends and the native laid paper:

France has proven to be many times more involved than I had ever imagined when I started, but I suppose this is true of many 'older' countries once you dive in.

The Germania issues of the German Reich are also quite complex, just because of all their color nuances in both types of printing (peace printing and war printing) and also because of the perforation variations and partial retouching in the Mark values (symbolic representations of the German Reich).

The Greek Hermes heads can be pretty confusing; they have many of the same problems as rednaxela (welcome!) describes with the Germanias, and "clean plates" and "not-so-clean plates" add to the confusion. I love the Hermes heads, and I like to think that I've got at least half of the ones in my collection identified correctly!

The overprints from the reclamation of the northern parts of Greece after the collapse of the Ottoman empire are a lot of fun, too.

I also really like postwar Germany, but PostmasterGS is right on the money with the provisionals and the AMG printings, which are among the sloppiest in all of German philatelic history. The 1948-51 series (Sc. 634-661) is so diffuse I decided that every single stamp must be its own variety.

I looked at some Indian states stamps once. I knew I'd met my match, so I went off and started a Teddy Bears on Stamps collection.

France is comparatively simple, although if one wants to get into the weeds, one can dive into semeuse varieties, or early plate positions. The only real headache I found was distinguishing among early mainline French stamps, Bordeaux provisionals, and French general Colonies. But in tackling that problem I stumbled into what is now one of my primary collecting areas: French General Colonies postmarks.

For the United States, it is harder to bypass the headaches. I am defining my US collection as face-different, so as to avoid minor differences in early stamps where Scott lists what should be varieties and plate positions as fully distinct stamps. Also, I am steering clear of coils and the whole Washington-Franklin mess. The Banknote paper varieties are harder to avoid, and I keep trying and giving up.

Canadian paper types also stopped me in my collecting tracks.