I don't think it was the best. That honor goes to Halo 2 which beats every other Halo in every campaign area, except for it being a bit too linear.
That being said: there is the main downside to ODST as well: it's too linear at times. Some levels were really great. The New Mombasa hub world for instance, or Uplift Resort and Coastal Highway. But then you also have shooting galleries like NMPD HQ and Kizingo Boulevard which weren't that much fun.
However, each map had it's great moments. The shaking of the "bridge" in NMPD HQ. The dark, mysterious vibe of Data Hive. The full blown invasion knocking at New Mombasa's door in Coastal Highway (still the only time we actually saw a true invasion in Halo!). The space elevator crashing down in Uplift Resort. The drop with the droppod. And the list goes on.
But apart from having tons of memorable moments, I think there are a few other elements that made ODST so great.
Firstly, it came at a time when a lot of fans were wanting something new to be done with Halo. H3 received a lot of criticism, like Reach gets now. And lets face it: H3 was a bit lackluster. And then came ODST, with more focus on story than H3 had (with that moving more into H2's territory). Which was smaller scaled and therefore easier to follow for a lot of people (taking cues from CE). And did something new, made Halo feel fresh again.
Not only that, but it also felt a bit like a throwback to classic Halo at times. No dual wielding. No epic, intergalactic space opera (which I do like, honestly), no uber jumps or feeling unstoppable.
Simply put: ODST has a lot of memorable moments, and came at the right time in the series. It's not the best (to me), but it's definately a great, great game.
Just too bad some people judge a game mainly on it's pricetag (which was just 40 euros/pounds in Europe...20 below full prized retail games)