Honest Hearts and old world blues iirc
tl;dr: That is correct. Buy those two.
Dead Money: Survival Horror in a hotel. A completely separate economy, invincible enemies, exploding collars, the works. If you get this you will DEFINITELY want to mod out some of the gameplay elements.
Honest Hearts: Mormons vs. Indians. This was my favorite DLC by a lot. The area's like visiting Yosemite, very different from the FO3/FONV desert. Best characters and writing in the game imo. <3 The Burned Man.
Old World Blues: "Goofy" sci-fi fallout. Very 50's B-movie vibe. This one is funny, for some values of funny. A lot of people's favorite.
Lonesome Road: Kind of a linear run-and-gun adventure; has some narrative conclusion about the Courier but otherwise not very well respected. At this point I was way burnt out and didn't even finish.