My ranking:
3>CE>Reach>2
2 wasn't a bad game, but a lot of the ways it tried to evolve from CE weren't fully realized until 3 which is why that one is the ultimate realization of Halo IMO.
Reach was an overall good game too, I actually loved the grittier art style. Long Night of Solace is one of my favorite levels in the series, as well. But aside from that level and the solid early levels, nothing was good enough to put it above anything else in the series save for 2. Also, lots of focus was put on the characters and story, but everyone except Jorge had no personality whatsoever, and most of the time I had no clue why I was even in a level.
The reason I left ODST out is because, as a spinoff, it wasn't bound by the Halo name. They would never have done the level hub, the highlighting HUD, the silenced weapons, the increased danger, etc. in a numbered sequel. It's probably my favorite, and a must-play, but if you can only play one Halo game it's not indicative of the series. It's best played alongside Halo 3, actually, which is basically how it was intended.
Also, Wars is actually a really great game too, but obviously it's even more far removed from the series than ODST. Unless you are a big Halo fan already or are just itching for a good strategy game on Xbox, there's not much reason to play it.
I'm a big Halo fan and I actually want to discuss how the Flood work from a gameplay perspective and why lots of people have come to dislike them (in my opinion unfairly), but I'll save it for when Alexis and Drew start encountering Flood again.
Bungie are working on a Marathon game, that does make me wonder what it'll be actually.
They were great games, better than Doom and Duke anyway.
And yes, there is one on XBLA, the second I believe.
It's my understanding that the project at Bungie was never officially stated to be Marathon related (though very strongly hinted to be), but was ultimately cancelled?
For anyone that still wants to play the games, the entire trilogy is now available for free much like the first two Elder Scrolls.