Alnico 2 traces back to the days of the 1950 Telecasters. This class seems to work well for middle-position pickups. It is known by its sweet, vintage tone, and there seem to be little disadvantages to using it.
Alnico 3 is the weakest of the three grades. It contains less cobalt, a core ingredient in the rest of the Alnico family. Because of the decrease in magnetic pull, it works wonders for neck pickups, where there is the most string vibration.
Alnico 5 is the strongest of the three alnico grades. The tone and response of the magnet is more powerful than its two cousins. This makes it perfect for bridge pickups, where there is the least vibration. If you want an aggressive, in-your-face kind of sound, Alnico 5 is your best bet.