hmmm i really don't know because all i've played was halo 1 and 2, so my pick would have to be those two games.

None. Halo is......inferior as a game in my personal opinion.

However, I don't judge anyone who likes the games.

The most fun I had playing Halo was playing online where I got to drive some teammates off cliffs.

Yeah I am the troll.

Lulz no. Trolling is taking a warthog, driving to the enemy base and having a foe jump in the gunner seat and take him back to your base to annihilate your own team.

Anywai, I'd gotta say so far, Halo 2 and 3. The single player were amazing and could try and hold itself up on that, then thinking back to all the hilarious times i've had online with friends.. yep. No others can compete.

Campaign wise I'd have to go with the first. I just REALLY like the concept of this superhuman thug and ridiculously powerful A.I. completely isolated on some giant world ring. Compared to the others where its more of an all out war. In Halo 1 I felt like I was reading a Heinlein novel, so suspensful and crazy twists, i.e. the flood!
Although I have to point out masterchief and cortana = Ender and Jane. And the human, covenant, flood races seem all so familiar... terran, protoss, zerg!?! Yea nice rip offs Bungie.
Multiplayer though its all about 2 and 3!

For me it's got to be Halo reach and Halo 3 

I've only played Halo 1, so by default it's the best Halo I've ever played. Everything after ODST looks like garbage thought.

I'd have to say 3 is my favorite but Reach is the best.

Halo 2 all the way across the sky!

Halo 3 was the only one where I got into the multiplayer (got bored of reach, single player only halo 1 and 2)
I was a boss at that game, and it was loads of fun playing with friends.

imho, I'd say Reach > 1 > 3 > 2

Never played ODST, though.