You could use a bit of python to get the ID (your identifer or OID@)
My dummy table of data.
flds = ["ID", "Station", "ChloroVal", "SelFlag"]
data_dict = {}
with arcpy.da.SearchCursor(tbl, flds) as inCur:
for row in inCur:
ID = row[0]
station = row[1]
val = row[2]
data = [val, ID]
if station in data_dict:
data_dict[station].append(data)
else:
data_dict[station] = [data]
data_dict looks like this :
{u'B1': [[5.4, 1], [3.4, 2], [6.4, 3], [5.0, 4]],
u'B2': [[3.2, 5], [5.2, 6], [5.0, 7], [7.0, 8]]}
then you read through this dict and sort each list.
for k, v in data_dict.items():
v = sorted(v)
data_dict[k] = v
The sorted list looks like this :
{u'B1': [[3.4, 2], [5.0, 4], [5.4, 1], [6.4, 3]],
u'B2': [[3.2, 5], [5.0, 7], [5.2, 6], [7.0, 8]]}
now all you need is the ID associated with the last member of the list.
updateList = []
for k, v in data_dict.items():
updateList.append(v[-1][1])
Then with updateList, open up an update cursor and set the selectFlag.