No airport that doesn't have its own train station could possibly be in the same sentence as the word "best", in my opinion.
Having said that, Terminals 1, 2 and 4 are pretty good, for what they are, and it's great to be able to walk between each of them. T3 is old and no longer fit-for-purpose. The passageway to the gates is so narrow that during peak times it's ridiculously crowded, and when boarding, the queues extend into that same passageway, making things even worse.
Re the arrivals experience: I fly a lot between MEL and SYD. At the baggage carousels in SYD, I almost always have my luggage within 4-5 minutes of getting there. In MEL, I generally get my luggage within 20 minutes.
So I agree with others: MEL could only be given "best" status if money changed hands somewhere along the track. I'd give it a 6 out of 10.