Alnico V and VIII magnet pickups are great for metal.
Alnico speaker magnets are usually used in low power settings lawyer & hipster stuff.
Hard to beat a high powered ceramic magnet speaker for guitar.
Alnico 9 is pretty good too. All else being the same, to my ears an A9 is like a blend of an A8 and ceramic.
A good while ago I did a magnet swap experiment on a set of Ibanez Infinity pickups I had laying around. I dug up what I wrote for results about the bridge pickup. Obviously there's more to things than just the magnet. There's wire gauge, number of windings, slug/bar, overall design, etc. to take into account. The INF pickups I believed were designed to be like a jack of all trades to cover cleans to high gain. So results may not hold true across every pickup, but it's a pretty decent general place to start.
The EQ is quantified on a scale of 1 to 10 with 5 being the midpoint having no boost or cut based on my perception. Output is based on a comparison between each magnet, but no hard numbers just my perception.
Alnico 2 - Low=5.5, Mid=7, High=5.5, Output=Moderate. Really nice mids, lows are pretty tight and rounded highs. It gives the pickup a really nice crunch tone with a lower vintage output. It reminds me of a PAF style pickup. Overall I say it's a good fit for that crunchy classic rock rhythm sound.
Alnico 3 - Low=5.5, Mid=6, High=6, Output=Moderate. Similar to the Alnico 2 in output. It has about the same low end as the A2 but with a little less mids and a bit more highs. It's also somewhat comparable to a ceramic magnet but with less output. This I think would work well for playing classic rock lead guitar.
Alnico 4 - Low=5, Mid=5, High=5, Output=High. This one seemed to be pretty flat across the board, no real boost or cut in any of the frequencies. Has more output than the A2 or A3 but not quite as much as the A8. Not quite sure of the best application for this one, maybe if you have a really nice amp and want to shape your tone with that instead of the pickup
Alnico 5 - Low=7, Mid=3.5, High=7, Output=Moderate. This one's the stock magnet and overall sounds kind of blah. It does have that scooped mids metal tone but just doesn't have the drive behind it to really be used for heavy metal. The output is more vintage and the tone is more metal and they just don't work well together.
Un-oriented Alnico 5 - Low=7, Mid=3.5, High=7, Output=High. If you're going for that scooped mids, high output metal tone this is where it's at. It has the same EQ as the stock A5 magnet and has the drive behind it to make it perfect for that chunky heavy metal rhythm sound.
Alnico 6 - Low=7, Mid=5.5, High=5.5, Output=High. I found this one to be kind of dark, not quite in the mud category but it's at least pulling into the driveway. The output Is good, similar to the A4 and UOA5. Still haven't figured out a good application for this one, maybe black metal. Actually it reminds me of if you put a bridge pickup in the neck position.
Alnico 8 - Low=6, Mid=7, High=6, Output=High. This one is an Alnico 2 on steroids. Great mids, tight bass & rounded highs. The tone is very similar to the A2 but with more modern/high output. It turns the INF2 pickup into a real beast. This would be perfect for those chunky heavy metal rhythm riffs. By far this one is my favorite.
Alnico 9 - Low=6, Mid=7, High=7, Output=High. I'd call this one a cross between the Alnico 8 & ceramic magnets. It has the low and mid response of the Alnico 8 along with the output combined with the nice highs of the ceramic. I think this would be perfect for balls to the walls high gain metal shredding.
Ceramic - Low=6, Mid=5, High=7, Output=High. Nice and bright with tight lows and moderate mids but not scooped mids. It keeps its articulation with high gain. The output is high but not as high as the stronger Alnico 8 or 9 magnets. It's your typical high gain lead sound but will also work well for rhythm.
