The major competitors for 'most powerful sniper rifles' are the Intervention and the Barret. Both seem to suffer from low fire rates, although the Intervention has less delay between scoped shots and the Barret can be hip-fired better. The Intervention also has a lower ammo capacity, but it has a much better reload-cancel.
The 'best silenced sniper rifles' go to the WA2000 and M21. If you were to silence all the sniper rifles (and, IMPORTANT, have Stopping Power on), the Intervention and Barret would kill in the exact way as the WA2000, with the WA2000 having the faster fire rate and lower recoil. The M21 can fire even faster and with even less recoil, but you stop being able to kill in the chest in one shot. It's a tough choice.
Unsilenced, the WA2000 is still a legitimate sniper considering you could opt out of using stopping power and still get a fairly powerful sniper that's better than an Intervention or a Barret without stopping power. I suggest hardline or cold-blooded.
And, yet again, the M21 is the best choice for Hardcore, because all sniper rifles kill the exact same way and thus the M21, with the lowest recoil and fastest fire rate, is the best one. If you want cold-blooded and a silencer the M21 is also your best bet, considering it has such a fast rate of fire and low recoil, but keep in mind you'll always need to take two shots (as with any silenced sniper without stopping power).
***Courtesy of thedawgXD on the Modern Warfare 2 boards on GameSpot for PS3***