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In terms of ground transportation to each cities downtowns as defined by Google Maps, here are the results (transit time only, not including walking), again provided by Google Maps. There may be more optimal routes and I'm sure different definitions of downtown, but this should provide a rough estimate in terms of which airports are conveniently located and accessible. Also added the new Silver Line in DC.

Train only
ATL to Peachtree Center - 18 min
MIA to Government Center Station - 18+ min (3 min MIA Mover and 15 min Orange Line)
PHX to Van Buren/Central Ave - 23+ min (6 min PHX Sky Train and 17 min Valley Metro Rail)
MSP to Warehouse District/Hennepin Ave - 26 min
PHL to Jefferson Station - 29 min
SFO to Embarcadero - 31 min
SEA to University St - 36 min
EWR to Canal Street - 39+ min (4 min AirTrain Newark, then 27 min Northeast Corridor, then 8 min E)
ORD to LaSalle - 45 min
JFK to Canal Street - 48+ min (9 min AirTrain JFK Green and 39 min A)
IAD to Farragut West - 49 min
DFW to Akard - 52 min

Train + Bus
BOS to State - 10+ min (5 min 66 bus, then 5 min Blue Line)
DEN to 16th Street Mall & California St - 47 min+ (37 min A Line, then 10 min MallRide)
LAX to Pershing Square - 50+ min (7 min Metro C Line shuttle, then 15 min Metro C Line, then 27 min Metro A Line, then 1 min Metro D Line)
IAH to Central Station Main - 52+ min (51 min 102 bus, then 1 min Red Line)

Bus only
CLT to College St & 3rd St - 24 min 5 bus
DTW to Woodward + Clifford SB - 59 min 261 bus

I decided not to do LAS, MCO, or FLL, since I'm assuming most people are not headed to their respective downtowns.