I have a few compound types I like. I think that the most battle efficient one would be steel compounding, because flying and shooting in bullet time is cool.
I, personally, would probably want duralumin compounding, even though a lot of it is still unknown. But being able to change your Connection to others (possibly learning their languages if it works like stormlight archives), perhaps becoming something like a gray man (wot reference) by tapping it and being able to take over koloss/kandra/inquisitors would be awesome.
My final favourite, the villain compounder, is zink. Imagine being able to outthink everyone while also controlling their emotions. Or just walking around with a dark cape and low grade spreading fear around you... And as a bonus, you would possibly even enjoy watching twilight if you were filling your metalminds at the same time!