Halo series has 7 games. I'm talking of the first person shooters here as there is another series called Halo Wars. Pick your top 3 games in the FPS Halo games and tell us why.

Halo: Combat Evolved. - The game that started it all and put the Xbox on the map.
Halo 2: The next step, defined console online gaming and put Xbox Live! on the map.
Halo 3: The height of the series that refined what the 2 prior games offered... But also offered so much more, cemented the Xbox 360 as a force not to be taken lightly.

1st: Halo 5
2nd: Halo 3
3rd: Halo CE

2,3,1

Halo Reach < Its dark sad story of a impossible mission to this day has me wanting more.

Halo ODST < Another dark Halo game where feeling human really made this a scray war to be in.

Halo 3 < Master Chief at his best. Heroics and the Arbiters best game.

I really like when Halo goes serious and dark. Hoping Halo 6 or the rumored Genisis goes the Reach/ODST direction.

Halo 4 - deepest and best single player in the series, it had me and my brother gripped all though the game.

Halo 3 - an amazing end of the first trilogy, great multi-player and an even better single player. Only downside is that the story isn't as involving as I would have liked, still great though.

Halo 5 - by far the best multi-player but weaker on the single player side. Still the Halo game I have sunk the most hours into, by a large margin

It's not that hard to make the full list with halo IMO (greatest to worst)
1. Halo 2 (solid campaign with the best multi)
2. Halo 3 (overall the best, everything is great but the community was peaking here)
3. Halo 1 ( like it for similar reasons to halo 3
4. Halo reach (a very strong campaign but the multiplayer took a big dive in quality)
5. Halo odst (it's ok I guess not very memorable)
6. Halo 5 ( weakest campaign of them all and a multiplayer that is just to crackbaby for my tastes)
7. Halo 4 ( ok campaign but a truly horrible horrible multiplayer)

Halo 2
Halo Reach
Halo CE

Halo CE
Halo 2
Halo CE Anniversary

The canceled Halo mega bloks would have been the best game in the franchise though

The original Halo is still my favourite, out of the ones I've played enough to rate (1,2, Reach) , I'd probably order them like this.

1. Halo CE

2. Halo Reach

3. Halo 2

Wasn't really a fan of the second one.