Chase quality: Campo, May 31, 2010. The perfect chase - open terrain, very slow motion, all aspects of severe encountered, topped off by one of the most photogenenic tornadoes of all time.
Meteorological significance: Although El Reno rings hard in my head (having, like many others, been there and being too near at some point), I have to go with April 27, 2011...the 'perfect' ingredients, at the high end of the CAPE/Shear diagram, was an extremely rare set-up.
Human impact: Again, El Reno is deep in my mind, as is Joplin (for a single tornado, this must be the top), overall I think April 27, 2011 tops it for me (a dubious accolade, of course, in this category). The sheer number of folk impacted, and also who were at risk on this historic day, means I think it is the top.