best screen Gamma?

[HowGudAmI]

what is better, on my iMac -

1.8 Gamma (default)

or

2.2 Gamma.

i think i prefer 2.2 - but which delivers the best image?

(if you dont know what im on about go to -
system prefs
displays
colour

then click calibrate and change the target gamma to 2.2)