I made a similar topic about this , but the opposite ...
i think Vice City & Liberty City are the best simply because Vice City had bright colors & it actually felt like the setting it was placed in , Also it had a really detailed map for 2002 IMO (VC 1986-VCS 1984)
Liberty City (Gta III 2001-LCS 1998 & Gta IV 2008) it seemed so alive and detailed & had outstanding maps in the 3D Universe & the HD Universe.
Those are my all time favorite maps out of the whole series.
What are yours?