Halo fucking 3 ofcourse. AI teambased combat.

I'm a big Reach fan.

Reach gets my vote. What I like most about the Reach campaign was the fact that every single level was amazing with no stuff in-between that you just wanna skip past and the story was great all the way through with plenty of emotion, There also wasn't any annoying levels like Cortana in 3 or the Library in 1 or even Gravemind in 2. Them levels mentioned just annoyed you from the get go and get worse with every replay you do of them.

I'm gonna be Mr Unpopular here and say I actually like Halo 4's campaign the best.

As cool as the Covenant are, after fighting them for four games, it was a breath of fresh air to have some new foes in the Promethean mechs. It also had some really awesome moments, like when you bring Infinity's weapons back online and get a front row seat from the outer hull as it unloads on the Didact, or the Death Star-esque sequence where you fly the Broadsword fighter across the surface of the his ship.

Plus, it was the only game in the series I've played that had an engaging story, with Cortana's deterioration due to rampancy and Master Chief having to be her rock while also struggling to cope with having a problem he couldn't solve with bullets or fists. Didn't hurt that it was also drop-dead gorgeous, with some of the best graphics of the last generation.

It had its shortcomings; it lacked the open playfields of prior titles and they could've been more creative with the Forerunner guns instead of sticking to the pistol/shotgun/machine gun/rifle template, but on the whole, I look back on its campaign with the fondest memories of the bunch.

I think its rather interesting how the Halo series is recongised. Master Chief is the face of Halo and the face of Xbox, however some of the series best games dont involve the MC at all.

It really shows how deep and veristile the Halo universe is. Reach and ODST are two of my favourite Halo games and the MC is absent in both, yet the MC is one of my all time favorite gaming characters.

Its strange.

I definitely like that game a whole lot. And you sell it very well.

Halo 4 has the best story by far IMO. It's an emotional roller coaster with the relationship towards Cortana and the Prometheans are a formidable Force to say the least. Also, we finally get an insight into the Forerunners and the war against the flood and humans. The story,coupled with a better movement system, simply astounding graphics and interesting levels make it a great SP. MP however...

The first one.

Reason: I've never played any other title, lol. =D

Halo 2 of course. Best physics, best ai, funnest on every single difficulty, good variety, and fun environments.

Some fun runs that I did:

1: Legendary difficulty, nuff said.
2: Vehicle only run, squishing vehicles into doors with grenades and stuff.
3: Melee only run, going as far as hitting flying enemies with only melee...

Cheers man. 

Halo 4 definitely has a different flavour to Bungie's entries, but I guess after four of those, by the time 2012 rolled around I was in the mood for a new take on the franchise.