There is probably much
more to be said about best coupling for maximum gain.

Maximum Gain, that seems to be all people think about.

Max. Gain is not always a good thing.

Example which is better for 2 meter mobile a 1/4 wave 0 dB whip a 3 dB 5/8
wave or a 6 dB collinear ?

A true 6 dB collinear makes a very poor mobile antenna.
Unless your parked on level ground, then it works fine.

The 3 dB 5/8 wave works good if you are on fairly flat terrain.
But get into the mountains or use repeaters that are on
10,000 ft. + mountains like we have here in WY and Colorado.
A 0 dB 1/4 wave whip can some times work better.

People put a 3 dB gain mag mount on the back corner of the
finder, setting at a 30 degree angle and wonder why it don't work as well as my
0 dB 1/4 wave mounted in the center of the roof of a van. (Mounted in a hole
in the roof, not a mag mount.)

All I am saying is there is more to an antenna than Max gain.
What about bandwidth, pattern etc.

There, I feel better.

73 Al Lowe N0IMW
Arrow Antenna