Currently working through the Point Lookout expansion on Fallout 3.
My favourite expansion of the lot.
I got all of them and enjoyed all of them, but that one is just all kinds of cool. And the rifle you get has become my favourite weapon.
I've only bought 3 of them so far; Broken Steel, Point Lookout, and The Pitt.
Broken Steel gets an automatic 'A' just for unlocking the end of the game and raising the level cap. It added some cool enemies too, like the Ghoul Reavers, and the Supermutant Overlords. The missions are ok, but I didn't find them overly challenging.
Am enjoying Point Lookout so far, although I've only done the first mission (helping Desmond defend the mansion), and have been spending most of my time exploring and killing mutant rednecks, which is fun in and of itself.
Not started The Pitt yet, will probably wait until I've finished Point Lookout first.