In general, "Ecology of..." and "Campaign Classics".
Specifically, a lot of the Dark Sun articles that ran back in the day were great. Although I've always enjoyed Dark Sun, it was not my favorite campaign back then, but many of the Dark Sun articles got photocopied, thrown in the boxed set and stuck around to become an indespensible part of the game.
To contrast, Forgotten Realms was my favorite campaign for much of my younger gaming years, yet F.R. articles were rarely of interest, in my opinion. I guess there was just too much of them. It often seemed like "oh great...another dragon, another bunch of magic swords, and other inn laced with wards."
I also really enjoyed the Forum section of the magazine, although with the advent of the internet and great places like this, I can see why it went away.