Halo 3 is my favorite one, but I barely touched 1, as I only got the anniversary edition right before the X1 collection announcement, and decided to play it there. And I never played the second one.
I didn't like reach, odst had some fun ideas but was lacking something. Halo 4 wasn't bad at all, better than reach for me.
Then again, I play halo for the campaign, online I'm pretty much only a gears player.