I'm hard pressed to identify a better lounge in the US including but not limited to the flagship lounges. Sure compare it to some of the international first lounges and you may find one you like more but in MIA there is no such animal.
I'm going to have to chime in and agree with abk. I have flown through MIA several times this year from Europe and usually poke my head in the Admiral's Lounge each time just to see how they are getting on with the big refurb. Last time (August), I heard a couple loudly complaining about the unflattering lighting and unwelcoming ambience of the entrance way ... Being the busybody I am, I volunteered that "it'll probably be much better once the refurb is completed" ... to which they replied in disgusted unison: "this IS the refurb!" ... *gulp. And that's before you get your little drink chit and stand around for your cube of pepper jack cheese and spoonful of ranch dressing (although I do rather like pepper jack and ranch)...
In MIA, the Centurion Lounge gives the feel of the much better option.