I'm no expert on this, but the most practical I've found so far is a small-radius hook,
like this.
Alpine Caving Techniques recommends a large-radius hook (
like this), but I like the smaller ones because they are easier to store or carry without being annoying, and the situations where I need it most seem to involve smaller holds anyway.
Black Diamond do an interesting
"triangular" skyhook, which I tried before getting the Reglette. It's meant to be more versatile, but I didn't really see much difference between the different hooks in practice.
With any skyhook, care is needed to ensure it doesn't smack you in the face when it peels off. Sometimes I keep one hand over the skyhook just in case.