I don't think there's a real "leader"...I mean, in terms of sales the cheap M-Audio stuff probably makes a killing.
Also, do you mean performance keyboards/controllers or digital keyboard workstations? The best workstation right now appears to be Korg's
Oasys - that thing is a monster. For performance keyboards, I think Nord is likely the leader in terms of quality.
http://www.korg.com/gear/prod_info.asp?A_PROD_NO=OASYS
http://www.nordusa.com/products.html