Let me add to Cowher's a little bit with some rankings:
Unranked: Bruce Arians
Although Arians coached under Cowher, he had DI head coaching experience and was an offensive coordinator at multiple locations. I feel he was what he was and didn't carve out a name for himself while in Pittsburgh while the others really established themselves as a viable candidates while here.
7.Dick LeBeau (Cincinnati: 12-33)
-LeBeau had a long history as a DB coach prior to the Steelers giving him a chance as DC where he got to make his mark. Obviously, he didn't have any success as HC for the Bengals but is still recognized as an innovator on defense. An even better coordinator than Dom Capers, who was terrific himself. He wasn't a good head coach, but a Hall of Fame DC.
6. Dom Capers (Carolina: 30-34; Houston: 18-46; Overall 48-80)
1 playoff appearance in 8 years; 1 division title; 1 NFC Championship game appearance
-Terrific defensive coordinator who got his first chance with the Steelers after a long history as a DB coach. Took over an expansion Carolina Panthers team and had some incredible success early on making the NFC Championship game his second year. Won a Coach of the Year in 1996. I'm not sure the Panthers took the correct approach to team building early on or he may have had some better success. He got another chance with the expansion Houston Texans but did not have any success. It probably didn't help their franchise QB never panned out. He's still in the league and I think is one of the best defensive coaches earlier in his career and it paid off into a long career. I wouldn't call him a complete failure as a HC, but he wasn't a success either.
5. Jim Haslett (New Orleans: 45-51; St. Louis: 2-10)
1 playoff appearance; 1 division title; 1-1 in playoffs
-Haslett had one year of experience as DC with New Orleans before taking over that position for three years with the Steelers. He was able to capitalize this into a HC job with the Saints. He only had one winning season but had an overall record of 45-51 with New Orleans. Moderate success as a head coach.
4. Mike Mularkey (Buffalo: 14-18; Jacksonville: 2-14; Tennessee: 20-21; Overall: 36-53)
1 playoff appearance; 1-1 in playoffs
-Promoted from tight ends coach to OC and did a pretty good job in that role. Got an opportunity with three different NFL teams as a head coach. In just over five seasons, he had three winning seasons. Not a particularly great career but did enough for teams to keep giving him a chance.
3. Chan Gailey (Dallas: 18-14; Buffalo: 16-32; Overall 34-46...Georgia Tech: 44-32)
2 playoff appearances; 1 division title
-Looking at Gailey's history, his coaching experience is fascinating. Prior to going to the Steelers he had experience as a HC, DC, ST coach, DB, WR, TE, and QB coach. Despite having head coaching jobs at Troy State, Birmingham, and Samford, it wasn't until his work as WR coach and then OC for the Steelers that capitalized into a HC job with the Cowboys. He was 18-14 in two years with two playoff appearances. He later had a decent stint with Georgia Tech and struggled in Buffalo. Moderate success as a head coach, but a better coordinator.
2. Ken Whisenhunt (Arizona: 46-51; Tennessee: 3-20; Overall: 48-71)
2 division titles; 2 playoff appearances; 4-2 in playoffs; 1 NFC Championship
A guy many wanted to take over for the Steelers after Bill Cowher retired, Whisenhunt was a terrific OC for us. Went to Arizona and did some incredible work early on with two playoff appearances and an NFC title in his first three years. The wheels fell off after that as Arizona remembered who they were and struggled with the QB situation. Tennessee was a disaster.
1. Marvin Lewis (Cincinnati: 131-122-3)
4 division titles; 7 playoff appearances
-Lewis didn't go directly from the Steelers to a HC job, but he made a name for himself in Pittsburgh as a LB coach. He earned a job with Baltimore under Ted Marchibroda and continued under Billick. HIs defensive philosophy had already been established by this point and did a good job in Cincinnati. Although he didn't win any playoff games he was a huge success as a head coach and probably should get another opportunity.