Ranked how? Number of houses sold? Number of listings sold? Volume (total prices) of homes sold? Highest commissions? Most transactions overall? Highest customer satisfaction?
Are you referring to a team, an individual or a brokerage?
I've never looked, but I'd bet that if you check the NAR web site they may have some rankings - usually in volume and transactions, but often these rankings don't tell you whether it is 30 agents working under one agent's name or 1 agent busting his/her hump. It's truly difficult to get an apples to apples comparison.
Generally speaking, on a national level, Keller Williams and ReMax have the agents that are considered top producers. Each of them may have published stats on who they consider the highest ranked agents, you can check their web sites, too.