I'm bending the rules a little bit here, but the best Sniper Rifle ultimately comes down to personal preference. Appearance, sound, ease/skill of use, where you started in the franchise — all of these factor into which sniper is your #1.
For me, I think the Halo: Reach sniper is my favorite. I know it's not everyone's choice, but I like the bass of the shot, the clicks of the ejection and reload, and the snappiness of a good headshot across a map.
Halo 4 and Halo 3 also have good snipers, and I think I like the sound of 4's is the best, but the total package doesn't do it for me.