What's the scale of satisfaction being measured? No point being the best of a bad bunch.
There are some great things about MEL - having three terminals under one roof is definitely one of them. The Ring+Ride and changes to the pickup areas is a step in the right direction, but not yet far enough given the volume of pickup traffic going through it. International T2 and Qantas' T1 are in relatively good shape, but Virgin's T3 (other than the Lounge) is in desperate need of refurbishment, and T4 is not even worth mentioning. And as
serfty mentioned, there are few kind words (other than 'café' and 'vue') that one could offer about the forced walk (no shortcuts, no travelators, no people movers) through the duty free area that sits in between security and immigration and the gates. The renovations to the international arrivals area have turned what was a perfectly acceptable entrance point for a major city into a cramped mess. Traffic is not great during peak, but it's relatively not bad and the new works being done should improve access to the freeway considerably. Domestic security is usually acceptable, but it is woefully understaffed at international security - despite all the infrastructure that's been added in the renovations, I have frequently been through at relatively busy times with only one lane in operation (rendering the express lane useless, unless you're prepared to be a DYKWIA, which sadly I have had to resort to on occasion for various reasons).
Having said all that, I'd still rate it above SYD which suffers from the terminal separation and the good-but-less-than-ideal methods for getting between them, a relatively worse position relative to the freeway, and the dreaded curfew. On the other hand, the proximity of gates and greater flight options make SYD more attractive for flying in and out. ADL and the new CBR are the only other Australian airports I rate, but I'd have to say they lack a bit of scale to compete against the others.
Just my 2c, seeing as I missed out on selection for the survey population.