Horse's hooves are made for dry environments & tend to do best in them. If your horse's hooves are looking dry & scaley, brittle, the grain is separated into 'micro cracks' or such, then IME it's likely nutritional. There could also be infection, mechanical or other factors too. So a good diet & nutritional supplement that balances his diet well should help. You wont usually see results from improved nutrition for a fair few months after starting it though.
Good quality ones - I find you get what you pay for & the 'cheap' ones aren't so cheap if they're not doing the job.
Depends on your horse's diet as to what he may need/want. Eg he may be low in only zinc, copper & iodine in his diet, so a 'hoof specific' supp may do, or he may be lacking/imbalanced in all sorts. Some mix & match, some prefer a 'complete' supp. If you're hard feeding him, a powdered supp can be easily mixed in, but if you're not & he can't afford a feed - already gets too many calories for eg. A low dose pelleted 'ration balancer' may be best. FeedXL.com is an economical & great service/program for working out the specifics & products that will fit your horse's 'bill'.