nice dark ones with any kind of stripping pattern on it, but usually rule of thumb is dark ones are the best, another thing you can do also is gently give it a SMALL delicate squeeze to see if its solid or not, bad ones will break immediatly and good ones can take that no problem
make sure it's not cracked or wrinkled. black or dark green/striped ones are best. press them between your thumb and finger and make sure it doesn't crack.