If it is by who sponsors the most legislation in the 113th Congress thus far, then it would be Sen. David Vitter (R-La.) and Rep. Alan Grayson (D-Fla.) for their respective houses.
If it is by whose legislation most often makes it through committee, that gets a bit tougher to figure out. Sen. Ted Cruz (R-Texas) has the highest 'batting average' with seven of the eight bills he sponsored having made it through committee. But Ron Wyden (D-Ore.) has had 13 pieces make it through committee, although he's sponsored 47 different pieces of legislation.
On the House side, it's even more confusing. Multiple congressmen have sponsored just a single bill, which made it through committee. Thus, they have 'batting averages' of 1.000.
All of this data comes courtesy of the Brookings Institute today in a fun Congressional Moneyball infographic they put together.
Here are the Senators who have sponsored the most legislation:
Brookings Institute
Here is the same ranking for those in the House:
Brookings Institute