Halo 1 was great when it came out but Halo 2 was just an overall better package with the excellent multiplayer for myself.
Halo 3 was pretty good too but still 2 is still my favorite.

Halo 2

I'll be damn pissed if 343 only brings a handful of Halo 2's multiplayer maps to the HD Anniversary.

2 > 1 > 4 > Reach > 3
(Haven't played ODST)

Played CE, 2, 3 and Reach. The first - Combat Evolved, is still my favourite. Mainly because of amount of time I spent playing local multiplayer with friends (so much fun). And it still has the best campaign and story in any Halo game, so there's that.

The game hasn't aged that well though, imo.

3>4>2>Reach>1>ODST

Halo 2.

I haven't played Halo 4, but Halo 2 is leagues beyond the others in my opinion. Fantastic multi-player with so many great maps.

Halo 2 for epic Multiplayer, Halo 3 or 4 for the epic story.

Halo 2 over LAN party was something else. But Halo 4 was the complete package to me.

1 and 2 are extremely close to me...