I'd say the top three ... in NO order ... Smart, Heupel, and (my apologies in advance) Kiffin.
* First, why do I not have Saban in the list? I think he is past his prime and I would have no interest in giving him a 4 year contract at his age (71). Reminds me of Tom Landry these days. Just so we are clear, but I had great respect for Tom! I also have great respect for Nick, but the last few years makes it look like he has lost a step or two. He is not on par with Biden, but I think I see the downhill slide unfolding.
* Not completely sold on Kiffin (48) as a HC (and I dislike him as a human - that might be affecting my judgement), but, as a coach, he does appear to be getting better. After all he has 17 years HC experience. He would be my selection if I was picking 3rd. [If he beats Alabama this year, he will be my odds on favorite to replace Saban when he retires at the end of this year.]
* The other two: Kirby (47) and Heupel (45) ... I just don't know about the relative 'value' of the other two coaches! I call it a toss up at this time. If I am picking 2nd, I take whichever one is still available.
Kirby (47) has been blessed with talent, but is his success due mostly to the abundance of talent at his disposal? Hard to say! Yet, let's give him credit being able to continue to bring in an overabundance of talent year over year. His location has a lot to do with the strength of his pool. The programs progression in 8 years is impressive.
Heupel (45) has done really well with less talent and his scheme. But he has only been a HC for 6 years. The progress of the program has been wonderful, the talent pool is improving, but not really closing the talent gap with those in from of him (according to me - but I certainly am not an authority on the topic). So, he appears to be doing more with less talent.
Final call ... Heupel ... if I was forced to pick first ... this would be more about my dislike for Kirby and hoping for greatness from Josh.