ODST doesnt deserve to have the Halo brand. It felt like the equivalent to the Halo series that CoD3 was to Call of Duty. The magic just isnt there. That being said I did enjoy Firefight.
I havent had a chance to play any of Reach surprisingly aside from the Beta test earlier this year but I hear it a step back towards the original.
JacDeathly is absolutely right when he said Halo 2 was the best. Storywise and Multiplayer it was incredible. Dont pretend like you didnt like the glitches and skulls. Did I mention how much of a badass I felt like playing as the Arbiter? Lastly I have to say, the Metropolis mission has to be in my Top 5 shooter levels ever. Mainly dude to the epicness of the Scarrab and Bridge fights.
That being said Halo 3 disappointed me greatly. Sure I loved it, but the magic just wasnt there. Campaign was lackluster at best and Online became a joke. Halo used to be the game that all us "serious" gamers played in LAN parties, not a whole bunch of 9 year olds who have yet to grow a pair. ODST was even worse IMO so much to the point I stopped playing the Campaign. It felt like something that could have easily just been an expansion or even another novel. And dont get me started on Halo Wars...
Bottom Line: Halo, while an incredible and iconic franchise(infact I would go so far to say the face of th4e last decade of gaming along with GTA and COD), has hit soem major bumps. Reach seems to be a step in the right direction but with installments like ODST and Wars, not to mention the ridiculously priced DLC, I cant help but wonder if all Microsoft cares about is getting money. I hope Frank O'Connor and 343 Industries turn this franchise around.