The rules: I had to have actually seen them live. No imagining from cast recordings. No bootlegs or PBS. Listed in (vaguely) chronological order.
  1. Linda Lavin in Gypsy
  2. Tyne Daly in Gypsy
  3. Donna Murphy in Passion
  4. Betty Buckley in Sunset Blvd.
  5. Emily Skinner and Alice Ripley in Side Show
    If the Tony committee can count their performances as one, so can I.
  6. Ann Reinking in Chicago
  7. Bebe Neuwirth in Chicago
  8. Tonya Pinkins in Caroline or Change
  9. Audra McDonald in Porgy and Bess
  10. Imelda Staunton in Gypsy
  11. Alice Ripley in "Next to Normal"
    Suggested by @erichutchinson