taken from espn.go.com
I'm a bit biased here, but Busch Stadium gets my vote for the best park. Although it can be technically classified as a "cookie-cutter" stadium, it is FAR more refined and aesthetically appealing than the other circular parks of the same era. Busch has it all: symmetrical, yet reasonable dimesions, grass turf, full manual scoreboard, no bad seats, and plentiful reminders of the heros of the past and present.
However, the best stadium should be recognized for more than just the edifice. The packed house and sea of Cardinal Red all season long reinforce the notion that Busch stadium the best park in the major leagues. http://espn.go.com/page2/s/list/ballparks/010427.html