Removing values from shapefile with ArcPy?

You can do this by looping through the shapefile first with a SearchCursor, creating a dictionary item for each "Key" field, where the value is a list of the "Accuracy" field values. Then loop through again with a UpdateCursor, compare with the maximum accuracy value from the dictionary, and delete the row as appropriate.

import arcpy

# Create dictionary
d = {}

# Loop through shapefile with Search Cursor
with arcpy.da.SearchCursor("myShapefile.shp", ["Key", "Accuracy"]) as sCur:
    for row in sCur:
        if row[0] in d:
            d[row[0]].append(row[1])
        else:
            d[row[0]] = [row[1]]    

# Loop through shapefile with Update Cursor
with arcpy.da.UpdateCursor("myShapefile.shp", ["Key", "Accuracy"]) as uCur:
    for row in uCur:
        if row[1] < max(d[row[0]]):
            uCur.deleteRow()