This question may be a bit more tricky than you first think.
Last season Cary Williams was the one who usually started the game against the other teams number one receiver. He actually fared quite well too. The Ravens defense often changes as the game goes on and Williams and the other corners would all end up splitting time between all the receivers. Williams however was usually matched with the top tier receivers in the beginning of the season.
Much of this may be due to the size of Williams. Standing over six feet tall he is one of the taller corners in the league. When the season started last year he was the tallest healthy corner the Ravens had. Most of the number one receivers in the league are over six foot, as is Williams. It would make sense to have height on height when you can.
On the other hand, statistically, Lardarius Webb was the Ravens best corner in 2011. He was also the biggest play maker in a secondary that features future Hall Of Fame safety Ed Reed and hard hitting Bernard Pollard. In most Ravens fans minds Lardarius is the obvious pick. But that does not mean he is ranked that way on the depth chart.
In fact, more often or not, you will find Webb matched up on a slot receiver. I personally think this is Webb's best position. Don't get me wrong, Webb can play both inside and outside, but I think he is most dominant in the slot. Especially when you have two other corners over six feet tall able to start on the outside. I think new defensive coordinator Dean Pees may feel the same way. He has had Webb lined up in the slot most of the time so far in OTA's. Even with Williams out of the line up, Danny Gorrer would play on the outside with Webb inside.
The Ravens most talented corner is Jimmy Smith however. Smith has all the tools to be a big time shut down corner for the Baltimore Ravens. Although you can not say he is the best corner on his own team yet, he could one day be one of the best in the NFL.
Smith had limited time on the field last season due to an injury suffered in the first game of the season. The Ravens chose to work him in slowly but his presence was felt when he was on the field. He was never really dominant but you could see the kid can play. He's one of those guys that always seems to be around the ball. I think you call those guys play makers. So Smith may not be the number one now but I think the Ravens see him as the number one of the future.