Milano Linate is the closest to the city and has different types of connections with the centre of Milan. From Linate airport, you can take bus number 73 (direction Duomo M1/M3) with an ordinary urban ticket Mi1-Mi3 (see GiroMilano ATM, Azienda Trasporti Milanesi) and reach the city center (Duomo) in roughly 50 minutes (please note that the bus is not convenient if you arrive/depart late in the evening or early in the morning). As an alternative, some buses can take you either to the city center or to the Milan central station.
Milano Malpensa is a little further away from Milan city centre, but it offers a larger number of international connections while being still well connected to Milan's central station with buses and a dedicated train (Malpensa Express).
Bergamo Orio al Serio also has buses services that connect the airport to Milan central station, but it’s the farthest from the city.