Alnico 2

Lots of mids, not much treble, loose low end, low output, and lots of vintage dynamics. Used in the bridge and neck.

Alnico 3

Like an A2 but with more treble and less low end. Pretty much used in the neck slot, but has potential for fattening up the bridge.

Alnico 5

Most popular alnico. Fairly high output, lots of treble and bass, scooped mids. Great in the neck slot (if not too bassy), and can be thin in the bridge.

All of these can be found as 'roughcast' too (lumpy pitted surfaces), that smooth out the high end, probably due to a more complex magnetic field. RC's are probably used mostly in the bridge location.

A2, A3, and A4 are all unoriented. A5's are almost always oriented. Unoriented A5's (UOA5) became available recently from Addiction FX and sound like a blend of the best characteristics of A2's and A5's.

They're much warmer than a typical A5, but with more treble than an A2. Lots of dynamics.