Malpenza Airport Milan to Centrale Station

I was in Milan only last month.
There is an airport shuttle you catch at the airport that will take you to the centrale station. Only seven euro. This is much easier than taking the train, because you don't have to carry your suit case up and down the stairs at the train station. I travel light, but I still find the shuttle much easier. They run very often, so if you miss one, you don't have to wait long for the next one. A taxi cost 85 euro, so that's not a good thing to do. Have a great trip!!
PS. but you did ask for the fastest way I just remembered. The train from the airport would be faster than the shuttle, so if you don't mind carrying your things down the stairs, you'll be fine. The train costs eleven euro, so still not very expensive. Here is the link for the shuttle...
http://ticketonline.malpensashuttle.it/inglese/pagine/default.cfm

You can take a train or a bus. The difference is that the bus will take you directly to Milan Centrale. If it's a time of heavy traffic, you could be slowed down. The train, on the other hand, is quite reliable, but goes to another train station, necessitating a subway transfer to Centrale.

If you need to arrive in Milano Cemtrale, the bus will be better for you.
http://www.sea-aeroportimilano.it/en/malpensa/index.phtml?mod=per_mi_scroll&to=come_arrivare

These companies run shuttles.
http://www.malpensashuttle.it/
http://www.autostradale.it/

From the Milan train station to the Metro Centrale Station means going up and down many more steps in and out of the metro.

No way to drop the rental car in Milan rather than MXP? Or somewhere else where you can get a train to Centrale Station?

What time of day? The trains and buses do not run all night.

BTW the bus is probably as fast or faster than the train because by train you still have to change to the metro, with luggage. Of course if you get stuck in rush hour traffic on the bus......