Tomlin IMO stepped into a perfect scenario. The Steeleers already had an established defense led by one of the best defensive minds of this generation in LeBeua. He had a playmaking QB in place and great personnel overall. I credit him for not screwing it up but there are other coaches that I would pick over him.
My list after BB would be:
1. Andy Reid. He hasn't won a SB, but he fields a very good team year in and year out. The Eagles went to the NFC championship game 3 straight years.
2. Jeff Fischer. It's not just his longevity, but during his tenure, the Titans are always a good team. Personnel has turned over a couple of times over there and the fact that they are still very good shows that his coaching has a major part of their success.
3. Shanahan. His offensive scheme and playcalling are top notch. He's proven that he can win with the right players. Not a great personnel man however.
4. Tom Coughlin. He took the Jaguars to the AFC championship game and they were a competitive team. Then proved it wasn't luck by coaching the Giants to a SB win. He was also successful at BC.
There are some other coaches that are coming up, but the guys I listed have been around a while and have proven themselves over time. If Belichick were to leave and one of the guys listed above were to take over, we know these guys field a competitive team.
There are other coaches who are up and coming but because they haven't been around long enough, I didn't include them:
Payton, Harbaugh, Rex, Whisenhunt, Sparano