XCOM: One Does Not Simply "Shoot" a Cryssalid...

[Anarion]

There's a key for that, and it is actually Y, despite the joke(?) about the X-Box controller.

On the PC, it varies to a different number key depending on what other abilities the unit has. Sometimes overwatch is 2, sometimes it's 3 or 4. Rarely 5+
This means that something like spamming "2" over and over won't work and you have to pay attention to each unit, taking up more time.

In my experience, the fastest method on PC is to double click the button and then press tab to go to the next unit right away.