So I decided to play through the entire Halo franchise since I had the MCC with ODST, and Halo Reach was a Games With Gold game. Plus I got Halo Wars for really cheap and Halo: Spartan Assault was a Games With Gold game on the XOne. I played them in release order, except for Halo Wars I played after all the others because honestly I'm not an RTS fan.
So after all that, I've determined that of the main games (6 if you count ODST as a main game, and by main I mean the FPS ones), I honestly thought Halo Reach was the best game. Granted I'm talking only single player but the single player experience IMHO was the best on it. The story was the best, and because they couldn't rely on Master Chief and Cortana's back and forth all game, it forced them to put other lesser known characters in front. I think Reach is terribly overlooked and underrated in the series, mainly because it doesn't have Master Chief.
To rank the others, the rest of the order would be Halo, Halo 3, Halo 4, and probably Halo 2. I throw ODST at the end simply because it's so short and more like DLC than a full game, but I will say playing it again 6 years later it was better than I remembered.
Halo Wars and Spartan Assault are spinoffs but they also were fun. Especially Spartan Assault. There is a sequel to it, Spartan Strike but it's not available on Xbox One or the 360 yet and I'm waiting for them to put it there. I'm assuming it will be ported later this year around the time Halo 5 comes out.