OP, and those struggling to nail down a good one..
Hoof Supplements at a Glance
Smartpak's chart was very helpful but my new farrier recommended Source Focus HF. I checked on the chart and it was really good in terms of price and what it has to offer. It complimented my horse's grain nicely (in terms of elements.) Don't want to overdue them!
:) Best of luck!
EDIT: OP it will take around ~6 months to see a significant difference in any hoof with any supplement. Just so you know!