The head of ER changed over the course of the show. William H. Macy (Dr. David Morgensterm) was in charge of both the ER and Chief of Surgery when the series began. Macy left the series partly over money but also because he wanted to film movies). In the show he has a heart attack in season four and has to step down.
Laura Innes (Kerry Weaver) steps in as head of the ER in season four. She becomes the hospital administrator in season nine and appoints Paul McCrane (Dr. Robert Romano) as chief of ER. Romano, short and ball, is by far the most despised ER chief because he is rather acerbic and doesn't care what people think. Romano is killed in season 10 when he is killed by a falling helicopter (a homage to an earlier incident when a helicopter blade took his left arm).
Sherry Stringfield (Dr. Susan Lewis) takes over as head of ER in season 11, but immediately regrets it as much because she and Dr. Innes don't get along, as well as a need to gain tenure (she lost her tenure track when she left the series the first time. Stringfield is replaced by Goran Visnjic (Dr. Luka Kovac), who runs ER for two years before leaving with Maura Tierney to become a hospice doctor.
Stanley Tucci (Dr. Kevin Moretti) is appointed head of ER but is so abrasive that no one likes him - when he takes a leave of absence to deal with his son of college, Kari Matchett (Dr. Skye Wexler) is named as his replacement - something that Moretti is surprised to find out. When Wexler decides she doesn't want the job, Angela Bassett (Dr. Catherine Banfield) is named the final chief of Emergency Medicine till the end of the series.
So here's your list of Chief of Emergency Medicine on the series ER:
1. William H. Macy - Dr. David Morgensterm
2. Laura Innes - Dr. Kerry Weaver
3. Paul McCrane - Dr. Robert Romano
4. Sherry Stringfield - Dr. Susan Lewis
5. Goran Visnjic - Dr. Luka Kovac
6. Stanley Tucci - Dr. Kevin Moretti
7. Kari Matchett - Dr. Skye Wexler
8. Angela Bassett - Dr. Catherine Banfield
You might be thinking of Anthony Edwards, who played Dr. Mark Greene. Greene never ran the ER but was the chief resident. George Clooney (Dr. Doug Ross) ran pediatrics. Other doctors who were on the show for a number of seasons included Noah Wylie (Dr. John Carter) and Eriq La Salle (Dr. Peter Benton).