A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

at best • at one’s best
at best—(introductory or parenthetical) even under the most
favorable conditions: He can only tie the score, at best.
at one’s best—(predic.) said of smb. in the best state or con-
dition: Although he was not at his best, he won the match.
at call • within call
at call —(also: on call) 
1
. (of a doctor, etc.) available if re-
quired: She served as president, then … twenty years as treas-
urer, always at call, and handling the money carefully. 2. (of
money, etc.) available on demand: As a first step, I secured my
vast property, so that the income would be certain, and always
at call.
within call —near enough to be summoned by calling: Ross
was still away on duty, and there was nobody except the native
servants within call.
at command • in command • on command
at command —said of smth. at one’s disposal (and ready to
be used): A constant supply of hot water is at command over
the whole house.
in command —
1
. having the authority to command: Like all
those who are seldom in command, the master was propor-
tionally tyrannical and abusive. 2. having control over a situ-
ation: He was looking more relaxed and in command than
ever before.
on command —at the order given: On command the mother
ship would have oriented itself and determined when to launch
its subsatellites.
at danger • in danger
at danger —(of a signal on a railway line, etc.): in the position
giving a warning of danger: A signal is said to be “on,” when
it is at danger.
in danger —at risk: The euphoria which accompanies alti-
tude sickness robs the pilot of the opportunity of recognizing
that he is in danger.
Note: 
The expression does not correlate in meaning with the
phrase on the danger list—(of a patient in the hospital) dan-
gerously ill: Five patients on the danger list were slightly better,
the hospital said.