Hmm... After some thought, I think my favorite all-time map that 
came with a game would be the 
Mission 5.3 - Crash Site: Confrontation[, from 
Perfect Dark (PD happens to be GameSpot's highest-rated FPS ever, BTW, with 9.9/10.0, while HL2 got a mere 9.4) The only disapointment I really had was that as well-fitting it would've been for a final mission, there were still 5 more levels yet to go!
![http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite1.jpg [http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite1.jpg]](http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite1.jpg)
I do think his life insurance premiums have gone up considerably...
![http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite2.jpg [http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite2.jpg]](http://img.photobucket.com/albums/v494/nottheking/Perfect%20Dark/CrashSite2.jpg)
Ugh... Skybox no worky, but level still look good, IMO.
It's a shame that this map isn't availible as a multiplayer for the game, though it is admitedly very large for one; a deathmatch could take over an hour here.
This was a bit of a tought call, when I had to think about the other levels; 
6.1 - Pelagic II has an amazing level of detail, and the mission is quite fun, even if it's a glorified rambo dash through the level while wearing a scuba suit. 
4.1 - Area 51: Infiltration, which I also posted a picture of for 
Name That Game, also happens to be rather nice. (I'd also like to see somebody wholeheartedly claim that the upcomming Xbox game called 
Area 51 has not been heavily influenced, if not completely spawned, by 
Perfect Dark's Area 51 mission.)
Also, 
1.3 - Datadyne Central: Extraction just has that 
Matrix-esque feel, the good part of it. There's nothing like scaling up through a skyscraper with barricaded hallways while fighting off droves of shotgun-wielding female bodyguards, and dodging that helecopter hovering outside the building trying to shoot you. (lots of glass shattering in the process)
As for the best add-on level to any game, that honor would hands-down have to go to 
The Dark Tide II: Nightfall, for 
Dark Forces. Patrick Haslow/Matt Hallaron, the makers of the level, posted shots for a JA multiplayer version of the level some time back, netting a SOTD or two. Unfortunately, I'm still currently fiddling with DF to get it to work on my nice new machine, so I can't post screenshots at this time.
        
     
Excelent choice. Glad to know somebody else plays DF.