My favorite DLC has to be the Pitt. It makes you play with none of your goodies for a while, having to rely on scavenging and sneak skills one stops using when loaded down with a full arsenal and power armor that deflects a nuclear strike. Also the moral question at the end is very compelling. Kidnap to set people free, or not and let a cure be discovered to help later generations. Thoughts?
I loved Point Lookout the best. For once, I didn't have to deal with any annoying rad scorpions and I always made sure to liberate the Blackwell House (at least that's the name I remember) for myself to act as a vacation house of sorts.
For whatever reason, I don't remember ever playing the DLC for Fallout 3 or New Vegas. I've seen people play through most of the DLC packs, but I never touched them myself.
My favorite DLC is the Pitt as well. Im really not a fan of DLC in general but decided to give this one a go and did not regret it.
I've only played that one once. I bet I missed out on all the good stuff too. I just did the main quests to get my piece of brain back.
Please, play them. There are some DLCs that are not really fun, and very linear, but they give you cool gear after you beat them. The ones you MUST play are Point Lookout, Broken Steel and The Pitt for Fallout 3 and Honest Hearts and DEFINITELY Old World Blues for New Vegas. Please play that, it's like a whole new experience. To the OP's question and the point of this thread, my favorite DLC in Fallout 3 definitely has to be Point Lookout. It gave me a different environment to play in, cool new weapons and different enemies. And, as a matter of fact, I took that ferry very early on and I was struggling a lot with those big fat enemies, but it just made my experience so much better. Because, for once, I felt like I'm not invincible. I loved the story of Point Lookout and everything about it. The Pitt is my second favorite Fallout 3 DLC, and it is amazing as well, but you asked for the favorite one, not the favorite two. However, loved that as well.
