I'm going to add the two (so far released) New Vegas add-ons to my list:
Dead Money: Where do I even begin with this? To put it simply: Dead Money is amazing and has stuck with me long after I left the Sierra Madre. Here I go, fangirling about Chris Avellone's writing but I swear the man has no equal when it comes to game writing and putting into words just what kind of power emotions hold. A lot of people seem to dislike having your equipment stripped in the beginning but I rather liked it, this forced me to actually try to survive the rather hostile environment and keep my eyes out for anything that would help me along the way. The companions, I simply adore all of them and in some ways it pained me to have to leave them at the end. I could keep going but the amount of positive feedback I have for this addon would be a small book. Yes there are things I could complain about (just two areas that I found rather difficult but it doesn't ruin it for me), but I walked away after playing it feeling so incredibly emotionally invested in the story, characters and location that it makes me wish I could go back ("but the hardest part, is letting go...")
Honest Hearts: While quite good, I have to admit I was a bit let down with it after playing Dead Money. I try to stay away from reviews until after I've beat something because I don't want my opinions colored in any way. I kept reading how HH was better than DM and for me personally, I politely but strongly disagree. From a storytelling standpoint I found myself connected to DM far more, and locationwise I didn't find Zion anywhere near as atmospheric as the Sierra Madre. I enjoyed it but I didn't feel the need to take my time on it. The characters were good, I was fond of the two tribal companions and the Burned Man is of course rather awesome but as a whole I walked away from it feeling not quite as impressed as I was with Dead Money.
I'm really looking forward to Old World Blues which is supposed to be out this month, and especially excited for Lonesome Road, given how much the other two add-ons hype it up.