I know opinions are going to vary here, but i just got the ultimate edition and have no knowledge of any of the DLC content. I had the GOTY edition of fallout 3 and my 2 fav dlcs were Mothership zeta and point lookout, the reason being : they were a much different enviorment than the wasteland. So with new vegas, is there anything like either of those? something that takes you away to a completely different type of enviorment?
Honest Hearts is much better at lower levels, if you go there at higher levels it is way too easy. The environment is very nice though and fun to explore. I consider the survivalist logs you find while exploring to be the main story of the DLC and not actual conflict you end up in the middle of.
Dead Money is probably my favorite, but other people really dislike it. It is by far the most divisive of the DLC and probably the most unique. It is better at higher levels because it has a lot of high skill checks. It has the most interaction with NPCs and use of non-combat skills of the DLCs.
OWB is the most well-rounded and popular of the DLC, it is good in a lot of ways, although not necessarily great in any, but has something for everyone.
Lonesome Road has the best high level combat and good gear, but otherwise there isn't a lot to it.
Old World Blues is easily my favorite of them-- it has some of the best writing & voice acting in the entire series as far as I'm concerned. Plus, if you liked MZ's 50s B-movies sci-fi aesthetic, you'll probably like it a lot.
However, since you are low level, I would recommend doing Honest Hearts first-- it is the easiest DLC & has a nice unique environment as well.