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

1. Halo 2
2. Halo 3
3. Halo 3: ODST

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