I have an Italian infantry force based primarily on GHQ figures, but bolstered by some very useful newer H&R Italian figures.
This is the core infantry company. My Italians are mostly in continental (gray-green) uniforms, so not optimized for the western desert campaigns. But the figures are the question here, not the painting.
This is a close-up of my Italian combat engineers.
The kneeling figures (with satchel charges) are H&R. So are the flame-thrower figures, and the LMG team. The majority of the riflemen are GHQ. You can see how nicely they mix.
H&R also makes some very nice guns. These are Italian 65mm infantry guns, with H&R kneeling gun crews. I don't know any other vendor that makes this gun in 6mm, and it was a relatively common (and useful) bit of kit to have around. I think it looks great -- first rate models.
But it is important to note these are all very new H&R figures.
I also have a fair quantity of older H&R infantry. I am not criticizing them here, as I have them, I use them, I like them. But they are different. The older H&R figures are of substantially lower detailing, and a bit smaller in size.
Here for example is a close-up of my US Army 81mm mortar company, comprised exclusively of older H&R figures. My entire US Army infantry force is older H&R stuff, and I am quite pleased with them. But they are not up to the level of the newer H&R stuff.
As others have noted H&R seems to offer the best variety of poses. Really useful. Adler comes in a close second in my book -- very useful and pleasing poses. But I find the Adler figures do not mix very well with other brands. They are pleasing and very nicely detailed and casted. But they are proportioned differently, generally stockier with notably larger heads in all cases I have seen. I have gamed with other players' Adler figures and don't object to the appearance. But I do not choose them for my collection.
GHQ individual infantry is extremely well detailed. Very nice figures. I have the Italian figures, but not British or American. However, it is very difficult to construct proper TOEs with the GHQ figures as they have rather oddly unbalanced mixes and are missing several key infantry weapons from almost all of their national choices.
Note with GHQ they also offer "company" packs (or other oddly named infantry backs), that are an older en-bloc style of moldings with 8 or 10 figures shoulder-to-shoulder in a single casting. These date back to the late 1980 / early 1990s. I do not use these figures, although I have used some of the heavy weapons (crews and weapons molded en-bloc). Fortunately GHQ provides pictures on their website so you can quickly identify the en-bloc castings.
Also there are a few GHQ infantry packs that are reputed to be notably large for 6mm scale. In particular the Vietnam infantry (US and Viet Cong), and the US WW2 Paratroopers have been shown in various fora to be closer to 8mm or even 10mm. I do not have any of these, and have not seen them in person, so can only suggest what I have seen discussed in other fora.
I have tried Scotia in the past. They are, to my experience, less consistent than some of the other vendors. Some of the figures are quite nicely detailed. Others … well I have trouble figuring out what they are supposed to represent. They are also proportioned more to the stocky side, and are notably larger than most of the GHQ, H&R new or old, or even Adler.
This pic is from when I tried to add some Scotia infantry to my WW2 French force. Here you see a Scotia figure (with binocs) next to an (older) H&R combat leader figure. The cast-on bases of the figures are about the same height, but the Scotia figure towers a full 1.5mm above the H&R figure. It is also somewhat stockier. Not necessarily a bad figure, but not the best choice if you might use multiple vendors to get the combinations you want in your force.
There are additional pics and discussions of infantry figures in my "Recent 6mm Work" thread.
Good luck and good gaming.
-Mark
(aka: Mk 1)