New Vegas easily a great game, and better than 3.
Now, let's get into why.
Firstly, the story is almost 200% better. The game is much longer, for one thing, with the main quest stretching 12 hours at the least. Secondly, the third very much railroaded you- you only have two options to finish the main quest, and there were only two endings for the main quest. New Vegas has a much better main storyline- I'm assuming you know it, but it is much, much more interesting. 3 reused the same factions from 1 and 2. New Vegas uses the NCR, but it makes sense, unlike the Enclave and Brotherhood in 3.
On top of this, the writing is much better, with plot holes being very few and far between. The game can be finished in 4 main ways, with tons of variation on top of the main 4. For example, what you do with the Fiends does effect the ending outside of their specific ending.
Though New Vegas's story is so, so much better than the third, there are a couple problems.
Well, not problems, but deficiencies, I would call them. For one, the engine is still as poor as it was in 3. Some improvements were made- you get stuck in objects much less often. I was resetting Three almost every hour because of getting stuck. As well, I believe the AI is better, though not much.
Beyond this, 3 seemed more stunning to me, visually. I played both on a 15 year old, crappy, non HD TV, and that does affect it- I've played New Vegas on a 40 inch HD plasma, and almost came my pants. But besides that, the Capitol Wasteland, however obnoxious it was to explore the inner city, seemed much more stunning to me. The color tint that gets bashed so much actually really helped for me. I won't lie- The Strip and the Grand Canyon were incredible for me, but nothing compared to walking onto the Mall for the first time, or looking out of the Washington Monument. The atmosphere of Three is really the only thing in it better than New Vegas.
Outside of those two deficiencies, New Vegas is one of the greatest games, story wise, I have ever played. I've only played the naked version of New Vegas, and if the DLCS are as good as Three's, I cant wait to play them. The engine does leave something to be desired, but hopefully the next game will be in the engine Bethesda used for Rage and Skyrim. If so, it'll top every game I've ever played, if the story is as good as Vegas.