The larger 'parrot' ravens are the simplest and probably the best for most applications. The smaller 'crow' type are much harder to find, and will usually be out of stock. They are also harder to keep with your minifigure unless your mini has a tiny hole for the crow to mount in (in the hair or hood?).
Our simple substitutes might not look like much, but they are better than nothing if your budget is tighter.
- Hills, Swamp, Urban