I've tried various LNBs and have stuck with the Inverto Black Ultra.
I had a Darkgold 0.1 and it was good at picking some frequencies up, but hopless for others. It was very different throughout the frequency range and some were very poor indeed.
Next best I found was a MTI, again was fine on some frequencies, but hopeless for others. But not as bad as the darkgold.
I got good results from a Technisat white 0.4, pretty even quality whatever the range and actually peaked with a few tps higher than the Inverto Black Ultra.
However, for the most consistantly good throughout all frequencies, whatever polarity I've found was the new inverto black Ultra. I had to buy it from Austria, availability in the UK is not good yet (couldn't find anywhere at the time).
But, this is only my opinion using my Fracarro Penta dish with my perticular set up. You may find very different results. Don't be taken in by low noise ratings, some might be low on a specific frequency, but not accross the full range.
I agree with Tivù, buy 2 or 3 different lnb's and find what works best for you.