My rankings:
1) DEN - straight forward terminal layout, moving walkways help with long walking distances
2) LAX - great UC, relatively small terminals make for quick connections
3) IAH - good terminal layout, easy connection if mainline to mainline, going to/from regional can be tricky
4) SFO - good food options, easy connection if domestic to domestic, long walk to get to the international terminal, prone to significant delays when visibility is reduced
5) ORD - crowded gate areas, crowded food courts, prone to delays when storms are present
6) EWR - crowded gate areas, crowded lounges (good luck getting in during peak times), train too small, delays are virtually guaranteed in the evenings
Haven't visited IAD.