Old World Blues was the bomb, no questions asked. And Dead Money was practically trash. I haven't finished the Lonesome Road DLC, but from what I've played of it it's okay. Can't forget Gun Runner's Arsenal, which was the bee's knees. Night-vision sniper-scoped varmit rifle all day every day, my man. Honest Hearts was interesting, but I'm not a huge fan of the whole tribal thing they had going. I'm still a huge fan of weird and wacky sci-fi, which is why I'm going to say again that Old World Blues is the greatest.
I removed the .com just to make sure that link didn't lead to a potentially harmful website, right now it appears to be a domain name for sale with ads on it -SiegeLion
Also you should totally finish Lonesome Road, the SMMG is real fun.