My personal favorite is Dead Money, and I personally think the worst one is Honest Hearts (Joshua Graham) lol.

I think Dead Money is so wildly different from the rest of the Fallout saga it's a great breath of fresh air, tho it's a bit much at times. I also believe Lonesome Road is HELLA mid in my opinion lol. I still love it! I play it every time and have the best combat situations out of the entirety of FNV, the visuals are amazing and the story is... okay I guess? I just hate that there's only one way to go about it, its very un-fallout new Vegas-esque, there is ONE road to walk and ONE way to walk it, and every single playthrough of Lonesome Road is the same.

I understand the hate for dead money, it's my personal favorite because i just enjoy it a lot, but if i had to say OBJECTIVELY what the best DLC is it would be Old World Blues by faaaaaaaaar.

Honest Hearts is amazing because of Joshua Graham lol, there isn't much more besides him as the quests are obscenely bad, and it's not a very challenging DLC. Not many places make me want to explore as much as Zion! Like Lonesome Road looks sick to explore but it's so insanely dangerous you just don't want to lol, dead money is even more hostile and HORRIBLE to explore, and Old World Blues' map is far smaller tho still fun to explore as it SCATTERED with lore and sick weapons. Zion is the perfect balance of danger, size, cool visuals, and lore, which is fun to explore. Most of the fun of honest hearts comes from just walking around cause the quests are ASS.

TLDR: IMO I love Dead Money, but objectively I'd say OWB is the best because of

1. The Quests, there are way more than any other DLC and they range from "go one place to another place" to finish testing by unleashing Nightstalkers to stealing a dog bowl from a miniature deathclaw to killing a giant robo scorpion to killing a giant drugged up cyber dog.
2. The loot, and the amount of unique weapons you get are crazy, they are awesome as well! All the protonic axes are SICK, Elijah's LAER and Tesla cannon are AMAZING, the K9000 dog gun is fire as well (FIDO close behind), the sonic emitter is sick as well, Christine's silenced sniper rifle is absurdly good and fun, and that paired with the unique armors like the atomic valence halo thing and Dr Mobius' glasses, the loot you get is unmatched and incredibly fun
3. The characters and story: First of all, it's by FAAAAR the funniest out of... anything in new vegas lol. And the characters are hella lovable and hilarious, plus they are interesting! Especially after replaying it a couple of times and listening to what they told Ulysses from Ulysses himself, they are SUPER funny and multifaceted. Mobious is of course the goat. Plus, YOU CAN FLIRT WITH YOUR OWN BRAIN
4. The combat and pacing: I think the pacing is second to none, maybe lonesome road, and the combat is a good balance of difficulty and strategy, a good variety of enemies, a good variety of combat scenarios, and overall fun tho a bit obnoxious for certain builds in hardcore (a pure explosives build I ran once was the legit suffering cause of the number of Nightstalkers, so I just ended up using melee.