It really depends. For splints I really like ProTech or Open fronts (More an english thing, but I like them)
BUT, for actually competing, I like Classic Equine Legacy boots, or SMB Elite VenTech. Though honestly I feel like the Legacy boots offer better support and protection.
I do love polos but they take ages to put on. And in the heat, putting them on and taking them off is a major pain and not worth it.