For me it's been that Inverto Pro single flange LNB (on a 1m commercial dish with matched feedhorn), and the Maverick LNBF on the 90cm GeoSat is a close second. My GeoSat Pro LNBF died last year....
I think that there are many 1.2m dishes made with different parameters, and they will perform differently with different LNBFs. But most of the antennas in the same size will perform more or less the same with different LNBFs. If you want better performance, just use a bigger dish.
For a 1.2 meter Amdrews or CM the feed that came with it and a Norsat LNB. I am lucky, mine was a dual polarity and just took the transmiter out and added two Norsats LNB's
I am running an Inverto Black Pro Twin on my CM 1.2m with great reception. But my $10 Maverick is nearly as good but I need twin output for my multiswitch.