What do you think is the best expansion ever made for a video game?

In terms of getting your money's worth, added content etc... Not so much having to do with how good the base game itself is.

Off of the top of my head, I'm thinking something like Beyond the Dark Portal for WC2 or Broodwar for SC. I think because essentially you're getting an entire new game that is (I think) larger, more difficult, and includes more units than the original.

I'm also enjoying Opposing Force for HL and Blue Shift for the first time, and I think they deliver.

My honorable mention would be Lord of Destruction for D2.

Heir to the Throne for EU3 for me. I haven't played any of those other ones though.

Shivering Isles.

Or the campaigns for Guild Wars. Though, while they are expansions by definition, they are really just whole new games (each one is standalone, meaning you don't need the original) that build off each other.

Morrowind's expansions.

Solstheim and Mournhold were places i enjoyed visiting long after each main quest was finished.

Diablo2:Lords of Destruction. imo, it's the main reason that D2 had the extreme longevity. That and a bunch of patches after, of course.
Dungeon Keeper:Deeper Dungeons - just more and better designed dungeons to play through
Stronghold:Crusaders - the setting wasn't as charming, imo (desert) but the gameplay was more interesting.
Majesty: The Northern Expansion - people wanted harder, they got harder.

I'm sure there's tons more, but that's all I'm coming up with offhand. I don't consider DLC's "expansions." Not yet, anyway.

Man, it's been a long time since I bought expansions for anything. Mostly because you don't see many of them these days. The only recent ones that come to mind are the Dungeons of Dredmor expansions, they're great. And Valve always provides good extra content, usually for free. Magicka's DLC that offer new adventures are okay.

Oh yeah, there's one that does stand out. Deus Ex Human Revolution - The Missing Link. A bit pricy on release, but still. It's a good 5 hours of quality content, and it handles the 'boss fight' at the end way better than the boss fights in the main game, which was the game's biggest flaw.

I suppose you could also count standalone expansions like Mount&Blade: Warband.

Further back, I liked Bloodmoon for Morrowind a lot, and even further back the Heroes III of Might&Magic expansions were good too.

GTA IV's DLCs.

Opposing Force
Blue Shift
Mysteries of the Sith

BioShock 2's Minerva's Den. A far better story than the main plot, and the closest a game has come to making me cry

For RTS, Yuri's Revenge for Red Alert 2 has the only single player campaign I thoroughly enjoyed from start to finish. Go back in time to fight dinosaurs, defend the London Parliament from a UFO invasion, run through a gauntlet maze of laser-shooting Moai statues, go to the moon where you can't train regular infantry or gather resources... there's just no beating it.

Shivering isles, it adds a whole fricking large new area of content.

Can I just go for the packaged deal of all of New Vegas' DLCs? No? Then I'll say Dead Money.

Shivering Isles.

The first four EQ expansions are what popped into my head: Ruins of Kunark, Scars of Velious, Shadows of Luclin, and Planes of Power.

Which is kind of odd because I didn't start seriously playing until LDoN. But I think I always saw those four expansions(especially the first three) as the core of the games and as representing Everquest at it's best, so yeah...

Oh, I gotta add Spearhead (and maybe Breakthrough) for MOH:AA.

Edit: And some more good ones were Aftermath & Counterstrike for C&C Red Alert, Covert Operations for C&C, and Duke it out in DC for Duke Nukem 3D.

Age of Empires Rise of Rome. Had a blast with this. Added a small number of things that made a big difference.

Quake 2 Ground Zero. Bigger, faster, badder. Just finished a replay this week of it.

Red Hammer + Resistance for Operation Flashpoint. Resistance in particular was great.

Opposing Force and Half-Life for me really ought to have switched places. Half-Life was the mediocre and bland experience while Opposing Force was the interesting and varied one.

Dark Crusade
Lord of Destruction
Frozen Throne
Thrones and Patriots
Bloodmoon

Does Opposing force count as an expansion?

Simcity 4: Rushhour
The Sims 2: Seasons/Nightlife (It's a tossup)
The Elder Scrolls IV: Shivering Isles
Rise of Nations: Thrones and Patriots
Half Life 2/3/?: Episode 2 (If it counts)
Age of Empires II: Age of Kings: The Conquerors

Warcraft 3 The Frozen Throne was an excellent expansion. Added so much to an already great game.

Yuri's Revenge for Red Alert 2 was excellent aswell.

The two Neverwinter Nights Expansions was quite good to.

All of the Expansions to Guild Wars.

Most notably my favorite would be Nightfall, though I know most useful (to me) was the Canthan one... (Ritualist and Assassin addition in an asian area, forgot exact name)

Civ IV beyond the sword

Or the Secret Armory of General Knoxx

Had a little think about this and I am going to go with Shivering Isles. Just in terms of what you can do and the well, madness of it all. Really loved it.

Final Fantasy XI
-Rise of the Zilart
-Chains of Promathia
-Treasures of Aht Urhgan
-Wings of the Goddess

Star Wars Galaxies
-Jump to Lightspeed
-Rage of the Wookies

Trials of Obi-Wan was pretty disappointing save for HK-47's return as a boss and the Old Republic lightsaber hilt that he drops.

Yuri's revenge.

Shivering Isles is awesome. Much better landscape and atmosphere than the original game, lots of content, and an interesting plot.

Dead Money was my favorite New Vegas DLC. Annoying at times ([censored] bear traps), but fantastic atmosphere, interesting characters and plot. Though I think I only liked it as much as I did because of a certain piece of fanfiction.