Asking which sniper rifle has the highest DPS is kind of a bad question, IMO. The most important question that you should be asking is, "How easy is it to hit successive headshots".
All the damage in the world isn't going to save you if you can't hit jack squat.
The M98 Widow had the highest DPS of any sniper rifle in the last game, and its probably still one of (if not the) best in this game also. The problem with this from a DPS standpoint is that it is single shot / reload.
The Black Widow (available from Spectre Requisitions) does almost the same amount of damage as a Widow, but it can fire multiple times without reloading, meaning it should do superior damage if you're more accurate. (You can also get multiple damage bonuses from cloak if you're fast enough, something impossible with the widow because of the reload).
You should also check out the Javelin, which is found on Rannoch. It's like the Widow, but focused on shields/barriers instead of just armor, which makes it very good for banshees.