Best train from Milan to Amalfi Coast (Positano)

Fast train Milano to Salerno or Naples
Then either Local train Naples to Sorrento bus or ferry or driver to Positano
Or
From Salerno-driver ferry or bus

No trains on AC

( Why do I get the feeling the whole world is going to the AC this year)?!

There are direct fast trains from Milan to Naples nearly hourly all day long. Check the trenitalia website for more information. Italo, the private train company, probably has some direct fast trains too. The train ride itself is close to five hours. Once in Naples you could ferry to Positano, take the local circumvesuviana train (a local train) to Sorrento and then ferry or take a bus to Positano, or hire a private driver to take you from the Naples train station to Positano. There are no trains on the Amalfi coast, buses, ferries, or private drivers. There are probably some other ways too, but it will be a long travel day. Could you stop along the way to break up the trip?

Like your other question, "best" is subjective to what is important to you. One thing to be aware of is it won't be simple and it will be a long day. Easiest but most expensive will be train to Naples then hiring a driver. Less convenient, but cheaper is a train to Salerno and then a ferry, or making your way to the Naples port and catching a ferry, or the local commuter train to Sorrento from Naples and then making your way to the port for a ferry. Keep in mind that if it's windy, the port in Positano may close and if you arrive late, the ferries don't run in the dark. If "best" means cheapest, then train to Naples, then train to Sorrento and then bus to Positano. But what you save in money you'll pay in chaos. The bus runs every half hour and you may wait 2 or 3 buses if they're full. And good luck with luggage if it's crowded.