In terms of pure numbers for this decade, its a tossup between March 12, 2006 and the May 2003 outbreaks.. However since many of these tornados were over hills and trees, chasers who frequent tornado alley will not remember these events so well (The one cell in the alley that comes to mind is the Kansas City Kansas F4 in 2003)
In terms of no terrain issues and being in the heart of tornado alley and a near perfect time of year for chasing purposes I'd go with May 29-30 of 2004.