Hey Duncan,

Most of my experience *playing* modded Minecraft (as opposed to developing) came in the first year or two after the Minecraft Coder Pack (MCP, made by Searge who now works for Mojang. It's development is an amazing story in itself) made source modding the game possible. I started creating my own mods about half-way through this timeframe. Here are the ones that stick in my memory, in roughly chronological order:

Mo' Creatures
This was the first mod I remember using, and one of the first to get popular from being spotlighted by big Youtubers (These were the days when the Yogscast, just Simon and Lewis at the time, were still the unchallenged kings of Minecraft on youtube). It was single player only, and simply added a lot of new animals and mobs to the game. It's looks like it's still being updated to this day.

BuildCraft and IndustrialCraft
These two mods arrived at about the same time, and remain standard in mod packs to this day. Between them, they pioneered the style of Minecraft mod that now defines Modded Minecraft: Automation. They were the first mods to add technological blocks that let you design and build automatic mining systems, factories and complex production lines in the game. I believe these types of mods to be the inspiration for full blown factory building games like Factorio and just-annouced Infinifactory.

Better than Wolves
This mod was also created to extend the Redstone and Automation features of Minecraft, but to do so in a way that felt more consistent with the stone-age/middle-earth technology level of the game. The name is a response to the addition of Wolves to the game in Beta 1.4. It later proved controversial as the author, FlowerChild, proved increasing hostile against what he saw as bad design in other mods, and Minecraft itself after Notch's departure, to the extent that he deliberately made his mod incompatible with others, and disabled several vanilla Minecraft features to keep the game balanced. It's very grindy, and has a very hardcore fanbase who stronly believe in it's design principles.

RedPower 2
This mod, at least in it's day, was probably the most well loved of the Automation mods. It adds a lot of simple components to the game that can be combined to make extremely complicated machines. Many of the blocks and items it adds, in particular Red Alloy Wiring and Bundled Cables, feel indispensable once you've used them. It's known for its very high quality level throughout: every texture is artful, every recipe is balanced, and nothing feels discontinuous with the rest of the game.

Those are a few of the mods that I've spent the most time playing, with the exception of my own (I'll leave it to others people to mention those). I'd also like to give a shoutout to the Portal Gun mod by iChun, which adds all components from Portal and Portal 2 into Minecraft. By design, it's not original, but it's really really fun.

I hope this list helps with your article, I really enjoyed writing it. Let me know if you have any more specific questions.

Dan