Inspired by the Opposing Force discussion in the other thread:
Quake: Dissolution Of Eternity
A lot of gamers tend to favour 'Scourge Of Armagon' as the superior mission pack but I found it a bit tiresome in places and enjoyed the second one a lot lot more. One of the main reasons is the whole Egyptian/Greek mythology thing, the levels are just so beautifully done for Quake and subtle changes abound with mummies instead of zombies and undead Pharoah demons. There are some really awesome puzzles and traps in this pack as well, most certainly my favourite expansion.
Doom 3: Resurrection Of Evil
I didn't like Doom 3 when I first played it, but I gradually warmed to it partly due to the awesome expansion pack that Posty's workplace produced. There was slightly more variation in environments, nothing huge but more outdoor areas made it into the game and there was a great section of riding the skytram back and forth to Phobos labs plus you had the artifact that more than made up for the loss of the soulcube 
Half-Life: Opposing Force
While cast as being a bit of a disappointment in the community in the early days, Opposing Force is a very well executed expansion that is more than worthy of being Half-Life's little brother. It's more of the same again but that's no bad thing and reinvented puzzles (especially Pit Worm's nest) work very well. The biggest attraction to me was always the fact that you could visit areas of Black Mesa that you only gimpsed in the original game , running around the Transit system whilst listening to the sound of a broken intercom was just awesome. The new weapons were very cool as well like the displacement cannon that you could use to warp to other areas to retrieve bonus items and more and the spore launcher which was a great way to get rid of meddling Black Ops 
I hated those things, they do a lot of damage in short time.
